The authors would like to thank all members of the SSGCID consortium. Achievement of the SSGCID's goals is made possible by the tremendous efforts of all team members at Emerald Bio. This research was funded under Federal Contract No. HHSN272200700057C from the National Institute of Allergy and Infectious Diseases, the National Institutes of Health and the Department of Health and Human Services.

<ol>
	<li>Lowen, A. C., Mubareka, S., Steel, J., Palese, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Influenza+virus+transmission+is+dependent+on+relative+humidity+and+temperature.">Influenza virus transmission is dependent on relative humidity and temperature.</a> PLoS Pathogens. 3 (10), 1470-1476 (2007).</li><li>Yamada, S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Biological+and+structural+characterization+of+a+host-adapting+amino+acid+in+influenza+virus.">Biological and structural characterization of a host-adapting amino acid in influenza virus.</a> PLoS Pathog. 6, e1001034 (2010).</li><li>Lorimer, D., Raymond, A., Walchli, J., Mixon, M., Barrow, A., Wallace, E., Grice, R., Burgin, A., Gene Stewart, L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Composer:+database+software+for+protein+construct+design,+codon+engineering,+and+gene+synthesis.">Composer: database software for protein construct design, codon engineering, and gene synthesis.</a> BMC Biotechnol. 9, 36 (2009).</li><li>Kabsch, W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Integration,+scaling,+space-group+assignment+and+post-refinement.">Integration, scaling, space-group assignment and post-refinement.</a> Acta Cryst. D. 66, 125-132 (2010).</li><li>McCoy, A. J., Grosse-Kunstleve, R. W., Adams, P. D., Winn, M. D., Storoni, L. C., Read, R. J. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Phaser+crystallographic+software.">Phaser crystallographic software.</a> Appl. Cryst. 40, 658-674 (2007).</li><li>Kuzuhara, T., Kise, D., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Structural+basis+of+the+influenza+A+virus+RNA+polymerase+PB2+RNA-binding+domain+containing+the+pathogenicity-determinant+lysine+627+residue.">Structural basis of the influenza A virus RNA polymerase PB2 RNA-binding domain containing the pathogenicity-determinant lysine 627 residue.</a> J. Biol. Chem. 284, 6855-6860 (2009).</li><li>Murshudov, G. N., Skub&#224;k, P., Lebedev, A. A., Pannu, N. S., Steiner, R. A., Nicholls, R. A., Winn, M. D., Long, F., Vagin, A. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=REFMAC5+for+the+refinement+of+macromolecular+crystal+structures.">REFMAC5 for the refinement of macromolecular crystal structures.</a> Acta Cryst. D. 67, 355-367 (2011).</li><li>Cowtan, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Recent+developments+in+classical+density+modification.">Recent developments in classical density modification.</a> Acta Cryst. D. 66, 470-478 (2010).</li><li>Chen, V. B., Arendall, W. B., Headd, J. J., Keedy, D. A., Immormino, R. M., Kapral, G. J., Murray, L. W., Richardson, J. S., Richardson, D. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=MolProbity:+all-atom+structure+validation+for+macromolecular+crystallography.">MolProbity: all-atom structure validation for macromolecular crystallography.</a> Acta Cryst. D. 66, 12-21 (2010).</li><li>Mossessova, E., Lima, C. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Ulp1-SUMO+crystal+structure+and+genetic+analysis+reveal+conserved+interactions+and+a+regulatory+element+essential+for+cell+growth+in+yeast.">Ulp1-SUMO crystal structure and genetic analysis reveal conserved interactions and a regulatory element essential for cell growth in yeast.</a> Mol. Cell. 5, 865-876 (2000).</li><li>Emsley, P., Lohkamp, B., Scott, W. G., Cowtan, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Features+and+development+of+Coot.">Features and development of Coot.</a> Acta Cryst. D. 66, 486-501 (2010).</li><li>Raymond, A. C., Haffner, T. E., Ng, N., Lorimer, D., Staker, B. L., Stewart, L. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Gene+design,+cloning+and+protein-expression+methods+for+high-value+targets+at+the+Seattle+Structural+Genomics+Center+for+Infectious+Disease.">Gene design, cloning and protein-expression methods for high-value targets at the Seattle Structural Genomics Center for Infectious Disease.</a> Acta Cryst. F. 67, 992-997 (2011).</li><li>Smith, E. R., Begley, D. W., Anderson, V., Raymond, A. C., Haffner, T. E., Robinson, J. I., Edwards, T. E., Duncan, N., Gerdts, C. J., Mixon, M. B., Nollert, P., Staker, B. L., Stewart, L. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+Protein+Maker:+an+automated+system+for+high-throughput+parallel+purification.">The Protein Maker: an automated system for high-throughput parallel purification.</a> Acta Cryst. F. 67, 1015-1021 (2011).</li></ol>

The authors are employees of Emerald Bio, Inc.

Multi-Target Parallel Processing
Structure-based drug design plays an important role in drug discovery. The SSGCID is dedicated to providing the scientific community with three-dimension protein structures from NIAID category A-C pathogens. Making such structural information widely available will ultimately serve to accelerate structure-based drug design.
The first critical step of the MTPP approach is construct design. Multiple constructs of each target protein increases the probability of successful structure determination and increases turnaround. It is inevitable that some protein constructs will fail during stages of the pipeline. Implementing the PIPE cloning method supports the MTPP method by allowing the generation of many constructs in 96-well format without labor intensive purification steps. Pairing PIPE cloning with the ability to analyze protein expression in the same 96-well format (Caliper LabChip 90) further expedites the overall flow. The pairing of these methods allows for quick identification of constructs that produce soluble protein which ensures the success of large-scale protein production and purification.
An essential aspect to the success of the MTPP high-throughput is the Protein Maker (US Patent No. 6818060, Emerald Bio) instrument. The Protein Maker is a 24-channel parallel liquid-chromatography system developed specifically to boost the efficiency of high-throughput protein production and related structural genomic pipeline research applications. Using the previously described protocol for the Protein Maker, the advantages are apparent in comparison to a single line FPLC system. A single person can purify up to 48 targets in parallel within an eight hour period. In contrast, a single person using a single line FPLC system can only purify a maximum of four targets within the same timeframe. The high levels of purity for each target achieved with the Protein Maker are a critical factor in the later success of growing protein crystals for structure analysis.
Limitations and Troubleshooting
Solving three-dimensional structures by x-ray crystallography is a multi-staged effort with many challenges, one of which is the inability to obtain large amounts of soluble target protein. One strategy that can be implemented to overcome the solubility problem is the use of an alternative expression host as E. coli cells are unable to perform several important eukaryotic post-translational modifications. Expression in various yeast, insect and mammalian cell lines that are capable of performing these post-translational modifications are often a suitable alternative. Target proteins are sometimes expressed but completely insoluble in the standard lysis conditions. The Protein Maker can be a valuable resource for the rapid testing of alternative cell lysis conditions as described in Smith et al. 2011 13. This strategy is often necessary to keep targets moving through the pipeline. In any structural genomics pipeline, standardized protocols may not be suitable for every target that comes through the pipeline and targets may need individual optimization. For example, we have chosen to use 20% ethylene glycol for every cryoprotectant. In cases that this condition is not suitable, alternative cryoprotectants or concentrations may need to be tested.
Due to the unique nature of each individual protein target, the rate-limiting and unpredictable step in determining a structure is crystallization. The MTPP pipeline offsets the commonly low success rate of protein crystallization with optimization from the initial sparse matrix screens. Each initial crystal hit from commercially available sparse matrix screens is further optimized with an E-Screen Builder (Emerald Bio). The optimization screen is designed around the condition of the initial crystal hit, altering the concentrations of the buffers, salts, and additives. Successful optimization screens yield crystals suitable for diffraction studies and structure determination.
The structural genomics program put forth by the National Institute of Allergy and Infectious Disease (NIAID) provides funding to Emerald Bio and three other Pacific Northwest institutions who together are the SSGCID (Emerald Bio, SeattleBiomed, the University of Washington and Pacific Northwest National Laboratory). Each member of the consortium was chosen for their expertise in applying state-of-the-art technologies required for accomplishing the goals of the NIAID structural genomics program. To date, SSGCID has deposited 461 structures into the PDB ranking it as the seventh largest contributor in the world, and in 2011, the most productive. The protocols and methodologies of the SSGCID are provided with the intention of benefiting the scientific community and perpetuating the research of infectious diseases.

<table border="1">
 <tbody>
 <tr>
 <td>Item</td>
 <td>Vendor</td>
 <td>Catalog number</td>
 </tr>
 <tr>
 <td>Primers</td>
 <td>IDT</td>
 <td></td>
 </tr>
 <tr>
 <td>Genes</td>
 <td>DNA 2.0</td>
 <td></td>
 </tr>
 <tr>
 <td>TE buffer</td>
 <td>Qiagen</td>
 <td>provided in kit</td>
 </tr>
 <tr>
 <td>96-well half skirt PCR plates</td>
 <td>VWR</td>
 <td>10011-248</td>
 </tr>
 <tr>
 <td>PFU Master Mix</td>
 <td></td>
 <td></td>
 </tr>
 <tr>
 <td>6X Orange Loading Dye</td>
 <td>Fermentas</td>
 <td>R0631</td>
 </tr>
 <tr>
 <td>10X TAE</td>
 <td>Teknova</td>
 <td>T0280</td>
 </tr>
 <tr>
 <td>Agarose</td>
 <td>Sigma-Aldrich</td>
 <td>A9414-10G</td>
 </tr>
 <tr>
 <td>pET28 vector</td>
 <td></td>
 <td></td>
 </tr>
 <tr>
 <td>2-YT Broth</td>
 <td>VWR</td>
 <td>101446-848</td>
 </tr>
 <tr>
 <td>Kanamycin</td>
 <td>Teknova</td>
 <td>K2151 </td>
 </tr>
 <tr>
 <td>Restriction Enzymes</td>
 <td>Fermentas</td>
 <td></td>
 </tr>
 <tr>
 <td>QIAquick Gel Extraction Kit </td>
 <td>Qiagen</td>
 <td>28704</td>
 </tr>
 <tr>
 <td>Top 10 chemically comp cells</td>
 <td>Invitrogen</td>
 <td>C4040-06</td>
 </tr>
 <tr>
 <td>Disposable Troughs (Sterile, 25 ml)</td>
 <td>VWR</td>
 <td>89094-662</td>
 </tr>
 <tr>
 <td>Airpore covers (Rayon films for bio cultures)</td>
 <td>VWR</td>
 <td>60941-086</td>
 </tr>
 <tr>
 <td>24-well blocks</td>
 <td>VWR</td>
 <td>13503-188</td>
 </tr>
 <tr>
 <td>QIAvac 96</td>
 <td>Qiagen</td>
 <td>19504</td>
 </tr>
 <tr>
 <td>BL21(DE3) cells chemcomp (phageR)</td>
 <td>NEB</td>
 <td>C2527H</td>
 </tr>
 <tr>
 <td>50% Glycerol</td>
 <td>VWR</td>
 <td>100217-622</td>
 </tr>
 <tr>
 <td>TB Media</td>
 <td>Teknova</td>
 <td>T7060</td>
 </tr>
 <tr>
 <td>IPTG</td>
 <td>Sigma-Aldrich</td>
 <td></td>
 </tr>
 <tr>
 <td>1 M Tris pH 8.0</td>
 <td>Mediatech</td>
 <td>46-031-CM</td>
 </tr>
 <tr>
 <td>5 M NaCl</td>
 <td>Teknova</td>
 <td>S0251</td>
 </tr>
 <tr>
 <td>Glycerol</td>
 <td>Aldrich</td>
 <td>G7893-4L</td>
 </tr>
 <tr>
 <td>CHAPS</td>
 <td>JT Baker</td>
 <td>4145-01</td>
 </tr>
 <tr>
 <td>Imidazole</td>
 <td>Sigma</td>
 <td>56749-1KG</td>
 </tr>
 <tr>
 <td>TCEP</td>
 <td>Amresco</td>
 <td>K831-10G</td>
 </tr>
 <tr>
 <td>L-arginine</td>
 <td>Amresco</td>
 <td>0877-500G</td>
 </tr>
 <tr>
 <td>Benzonase</td>
 <td>EMD</td>
 <td>70746-3</td>
 </tr>
 <tr>
 <td>Lysozyme</td>
 <td>USB</td>
 <td>1864525GM</td>
 </tr>
 <tr>
 <td>10 kDa MWCO dialysis tubing</td>
 <td>Thermo</td>
 <td>68100</td>
 </tr>
 <tr>
 <td>Amicon Ultra 10 ka MWCO concentrators</td>
 <td>Millipore</td>
 <td>UFC901024</td>
 </tr>
 <tr>
 <td>HisTrap FF columns</td>
 <td>GE</td>
 <td>17-5255-01</td>
 </tr>
 <tr>
 <td>HiTrap Chelating columns</td>
 <td>GE</td>
 <td>17/0408-01</td>
 </tr>
 <tr>
 <td>Compact Jr crystallization plates </td>
 <td>Emerald Bio</td>
 <td>EBS-XJR</td>
 </tr>
 <tr>
 <td>Crystalization screens</td>
 <td>Emerald Bio</td>
 <td></td>
 </tr>
 <tr>
 <td>Ethylene Glycol 100%</td>
 <td>Emerald Bio</td>
 <td>EBS-250-EGLY</td>
 </tr>
 <tr>
 <td>Crystal Wand Magnetic Straight </td>
 <td>Hampton Research</td>
 <td>HR4-729</td>
 </tr>
 <tr>
 <td>Mounted CryoLoop 0.1-0.2 mm</td>
 <td>Hampton Research</td>
 <td>HR4-955</td>
 </tr>
 <tr>
 <td>ALS style puck</td>
 <td></td>
 <td></td>
 </tr>
 <tr>
 <td>Puck Wand</td>
 <td></td>
 <td></td>
 </tr>
 <tr>
 <td>Bent Tongs</td>
 <td></td>
 <td></td>
 </tr>
 <tr>
 <td>Puck Pusher</td>
 <td></td>
 <td></td>
 </tr>
 </tbody>
</table>

multi-target parallel processing approach for gene-to-structure determination of the influenza polymerase pb2 subunit

Pandemic outbreaks of highly virulent influenza strains can cause widespread morbidity and mortality in human populations worldwide. In the United States alone, an average of 41,400 deaths and 1.86 million hospitalizations are caused by influenza virus infection each year 1. Point mutations in the polymerase basic protein 2 subunit (PB2) have been linked to the adaptation of the viral infection in humans 2. Findings from such studies have revealed the biological significance of PB2 as a virulence factor, thus highlighting its potential as an antiviral drug target.
The structural genomics program put forth by the National Institute of Allergy and Infectious Disease (NIAID) provides funding to Emerald Bio and three other Pacific Northwest institutions that together make up the Seattle Structural Genomics Center for Infectious Disease (SSGCID). The SSGCID is dedicated to providing the scientific community with three-dimensional protein structures of NIAID category A-C pathogens. Making such structural information available to the scientific community serves to accelerate structure-based drug design.
Structure-based drug design plays an important role in drug development. Pursuing multiple targets in parallel greatly increases the chance of success for new lead discovery by targeting a pathway or an entire protein family. Emerald Bio has developed a high-throughput, multi-target parallel processing pipeline (MTPP) for gene-to-structure determination to support the consortium. Here we describe the protocols used to determine the structure of the PB2 subunit from four different influenza A strains.

The following results illustrate the expected outcomes of the described protocol, and in the case of PB2, the observed outcomes.
Using Gene Composer, five full-length target amino acid sequences of the influenza virus polymerase subunit PB2 were designed (Figure 2). The PB2 sequences were back translated and subjected to many engineering steps 3, resulting in codon harmonized sequences optimized for expression in E. coli. From the iPCR products (Figure 3b), a total of thirty-four constructs were successfully cloned into a modified pET28 vector system 10 with an N-terminal 6x His-Smt fusion tag using PIPE cloning 3 as shown in Figure 3a. A summary of the cloning workflow is presented in Figure 4.
After successful cloning, micro-scale protein expression of each construct was tested in BL21(DE3) E. coli cells. Cells were grown in TB medium supplemented with Novagen Overnight Express 1 medium (according to the manufacturer's protocol) for 48 hr at 20 &deg;C in a shaking incubator set at 220 rpm. After growth, cells were harvested and tested for soluble protein expression using capillary electrophoreses with a Caliper LabChip 90. Fourteen of the thirty-four PB2 constructs led to soluble target protein and entered large-scale fermentation. Large-scale cultures of each construct were grown in TB medium supplemented with Novagen's Overnight Express 1 medium according to the manufacturer's protocol. After growth, cells were harvested via centrifugation and stored at -80 &deg;C. Large-scale protein expression of each culture was confirmed via SDS-PAGE analysis (Figure 5) before proceeding with large-scale purification.
The Protein Maker was used to conduct parallel purification of the fourteen PB2 constructs. The clarified lysates of all fourteen constructs were run through a nickel-chelate column. After determining which fractions contained target protein by SDS-PAGE, the corresponding fractions were pooled for each sample and the concentration of each was determined by an A280 reading. Removal of the 6x His-Smt tag was conducted by the addition of ULP1 followed by overnight dialysis and a second nickel column. Confirmation of the His-Smt tag removal was conducted by SDS-PAGE (Figure 6), and each sample was concentrated with a 10 kDa Amicon Ultra centrifuge tube. After concentration using the Amicon Ultra centrifuge tubes, each sample was run over a sizing column to achieve crystallographic purity. A second concentration was conducted to increase the protein concentration to a level necessary for crystallization. All fourteen constructs were successfully purified and entered into crystallization trials.
Crystallization was initiated by thawing the previously frozen protein. Crystallization was performed in a climate controlled room at 16 &deg;C with specially designed plates (Emerald Bio) for sitting drop vapor diffusion (Figure 7). Initial screening was conducted with four sparse-matrix screens; JCSG+, Pact, Wizard Full, and CryoFull (Emerald Bio), following an extended Newman strategy. 0.4 &mu;l of protein solution was then mixed with 0.4 &mu;l of crystallant (or reservoir solution) from the corresponding reservoir using 96-well Compact Jr crystallization plates (Emerald Bio). Of the fourteen purified samples nine of them yielded crystals suitable for diffraction studies (Figure 8). An in-house diffraction data set was collected on five of the nine constructs crystallized at Cu K&alpha; wavelength using a Rigaku SuperBright FR-E+ rotating-anode X-ray generator equipped with Osmic VariMax HF optics and a Saturn 944+ CCD detector (Figure 9). Each data set was processed with XDS/XSCALE4 and scaled to a final resolution. Attempts to solve the structures by molecular replacement were carried out with Phaser 5 from the CCP4 suite 7. The final models were obtained after refinement in REFMAC 7 and manual rebuilding with Coot 11. The structures were assessed and corrected for geometry and fitness withMolProbity 9. A total of four structures of the PB2 subunit were determined (Figure 10) and deposited into the PDB. Figure 11 illustrates the overall outcome at each stage in the MTPP pipeline.
<img alt="Figure 1" fo:content-width="5in" fo:src="/files/ftp_upload/4225/4225fig1highres.jpg" src="/files/ftp_upload/4225/4225fig1.jpg" /> 
 Figure 1. Overview of the SSGCID gene-to-structure pathway for Multi-target parallel processing at Emerald Bio.
<img alt="Figure 2" fo:content-width="5in" fo:src="/files/ftp_upload/4225/4225fig2highres.jpg" src="/files/ftp_upload/4225/4225fig2.jpg" /> 
 Figure 2. Alignment Viewer and Protein Construct Design Module in Gene Composer software. Amino-acid base construct of target is shown in green (middle window) and the structure guided truncations of alternative constructs are shown in gold (bottom window). An alignment of multiple Flu viral PB2 sequences is shown compared to the sequence and secondary structure elements of the C-terminal domain from PDBID 3CW4. Knowledge of the domain structure and secondary structure elements allows N-terminal truncations to be chosen within the Gene Composer Design Module by right-clicking on the desired amino acid residue. <a href="https://www.jove.com/files/ftp_upload/4225/4225fig2large.jpg" target="_blank"> Click here to view larger figure</a>.
<img alt="Figure 3a" src="/files/ftp_upload/4225/4225fig3a.jpg" /> 
 Figure 3a. PIPE cloning is illustrated wherein the synthetic gene insert (orange) is amplified by designed forward (red-orange lines) and reverse (orange-blue lines) primers to generate insert PCR material. The expression vector is amplified with reverse (red-black lines) and forward (blue-black lines) primers to generate vector PCR material. The terminal sequences iPCR products are complementary to the terminal sequences of vPCR products (red of iPCR complements red of vPCR and blue of iPCR complements blue of vPCR). This allows the iPCR and vPCR products to anneal to form plasmids that are replicated upon transformation into host BL21(DE3) chemically competent E. coli cells.
<img alt="Figure 3b" src="/files/ftp_upload/4225/4225fig3b.jpg" /> 
 Figure 3b. Agarose gel analysis of iPCR products from the PB2 subunit. iPCR failures may be seen as faint or smeary bands, while successful iPCR products are represented by robust bands. iPCR product quality can generally be correlated with cloning success. Molecular weight markers are in kiloDaltons. Figure is reproduced from Raymond et al., 2011 12.
<img alt="Figure 4" src="/files/ftp_upload/4225/4225fig4.jpg" /> 
 Figure 4. Gene engineering steps of target PB2 proteins were performed using Gene Composer software. After the engineered nucleic acid sequence was established for each target, 6-7 alternative protein constructs were designed for each. Multi-target parallel processing in the initial steps of gene design and cloning resulted in 34 constructs, 14 of which were viable targets that produced soluble proteins in E. coli.
<img alt="Figure 5" src="/files/ftp_upload/4225/4225fig5.jpg" /> 
 Figure 5. Representative SDS-PAGE analysis of large scale fermentation showing robust protein expression (expected size of 25.76 kDa), roughly 50% soluble (lane 4) and about 50% cleavage of 6x His-Smt tag from eluted protein (lane 7).
<img alt="Figure 6" src="/files/ftp_upload/4225/4225fig6.jpg" /> 
 Figure 6. SDS-PAGE results for three constructs of the polymerase PB2 subunit. Lane 1, molecular-weight markers (labeled on the left in kDa); lanes 2, 6, and 10, pooled protein from Nickel 1 column; lanes 3, 7, and 11, flow-through of cleaved protein in buffer A from Nickel 2; lanes 4, 8, and 12, removal of 6x His-Smt tag in buffer B from Nickel 2.
<img alt="Figure 7" fo:content-width="4.5in" fo:src="/files/ftp_upload/4225/4225fig7highres.jpg" src="/files/ftp_upload/4225/4225fig7.jpg" /> 
 Figure 7. A schematic of vapor diffusion by the sitting drop method. The sitting drop method for protein crystallization falls under the category of vapor diffusion. This method entails a purified sample of protein and precipitant to equilibrate with a larger reservoir containing similar conditions in a higher concentration. As water vaporizes from the protein sample and transfers to the reservoir, the precipitant concentration increases to an optimal level for protein crystallization.
<img alt="Figure 8" src="/files/ftp_upload/4225/4225fig8.jpg" /> 
 Figure 8. Protein crystal of polymerase PB2 subunit from a strain of the influenza virus.
<img alt="Figure 9" src="/files/ftp_upload/4225/4225fig9.jpg" /> 
 Figure 9. X-ray diffraction image of the polymerase PB2 subunit from a strain of the influenza virus.
<img alt="Figure 10" fo:content-width="4in" fo:src="/files/ftp_upload/4225/4225fig10highres.jpg" src="/files/ftp_upload/4225/4225fig10.jpg" /> 
 Figure 10. Ribbon diagrams of the molecules in the crystallographic asymmetric unit of 4 PB2 structures. Secondary structures colored in rainbow pattern with corresponding PDB codes. (a) 3K2V (A/Yokohama/2017/2003/H3N2) (b) 3KHW (A/Mexico/InDRE4487/2009/H1N1) (c) 3KC6 (A/Vietnam/1203/2004/H5N1) (d) 3L56 (A/Vietnam/1203/2004/H5N1).
<img alt="Figure 11" fo:content-width="6in" fo:src="/files/ftp_upload/4225/4225fig11highres.jpg" src="/files/ftp_upload/4225/4225fig11.jpg" /> 
 Figure 11. Outcome analysis for influenza PB2 targets by the methods described. The structure determination pipeline is illustrated in five steps: Cloning, solubility, purification, crystallization and structure determination.

Watch this Scientific Journal Video about Multi-target Parallel Processing Approach for Gene-to-structure Determination of the Influenza Polymerase PB2 Subunit at JoVE.com

Multi-target Parallel Processing Approach for Gene-to-structure Determination of the Influenza Polymerase PB2 Subunit

Structure-based drug design plays an important role in drug development. Pursuing multiple targets in parallel greatly increases the chance of success for lead discovery. The following article highlights how the Seattle Structural Genomics Center for Infectious Disease utilizes a multi-target approach for gene-to-structure determination of the PB2 influenza A subunit.

Pandemic outbreaks of highly virulent influenza strains can cause widespread morbidity and mortality in human populations worldwide. In the United States ...

multi-target-parallel-processing-approach-for-gene-to-structure

Research

JoVE Journal

Immunology and Infection

13.0K Views.  Emerald Bio. Structure-based drug design plays an important role in drug development. Pursuing multiple targets in parallel greatly increases the chance of success for lead discovery. The following article highlights how the Seattle Structural Genomics Center for Infectious Disease utilizes a multi-target approach for gene-to-structure determination of the PB2 influenza A subunit.

Video: Multi-target Parallel Processing Approach for Gene-to-structure Determination of the Influenza Polymerase PB2 Subunit

Using Bioluminescent Imaging to Investigate Synergism Between Streptococcus pneumoniae and Influenza A Virus in Infant Mice

During the 1918 influenza virus pandemic, which killed approximately 50 million people worldwide, the majority of fatalities were not the result of infection with influenza virus alone. Instead, most individuals are thought to have succumbed to a secondary bacterial infection, predominately caused by the bacterium Streptococcus pneumoniae (the pneumococcus). The synergistic relationship between infections caused by influenza virus and the pneumococcus has subsequently been observed during the 1957 Asian influenza virus pandemic, as well as during seasonal outbreaks of the virus (reviewed in 1, 2). Here, we describe a protocol used to investigate the mechanism(s) that may be involved in increased morbidity as a result of concurrent influenza A virus and S. pneumoniae infection. We have developed an infant murine model to reliably and reproducibly demonstrate the effects of influenza virus infection of mice colonised with S. pneumoniae. Using this protocol, we have provided the first insight into the kinetics of pneumococcal transmission between co-housed, neonatal mice using in vivo imaging 3.

Using Bioluminescent Imaging to Investigate Synergism Between Streptococcus pneumoniae and Influenza A Virus in Infant Mice

A concurrent infection with influenza A virus is one of the factors implicated in the induction of invasive pneumococcal disease during asymptomatic Streptococcus pneumoniae carriage. Here we describe a mixed infection method using infant mice to investigate the synergism between these two respiratory pathogens.

During the 1918 influenza virus pandemic, which killed approximately 50 million people worldwide, the majority of fatalities were not the result of ...

ampliPHOX Colorimetric Detection on a DNA Microarray for Influenza

DNA microarrays have emerged as a powerful tool for pathogen detection.1-5 For instance, many examples of the ability to type and subtype influenza virus have been demonstrated.6-11 The identification and subtyping of influenza on DNA microarrays has applications in both public health and the clinic for early detection, rapid intervention, and minimizing the impact of an influenza pandemic. Traditional fluorescence is currently the most commonly used microarray detection method. However, as microarray technology progresses towards clinical use,1 replacing expensive instrumentation with low cost detection technology exhibiting similar performance characteristics to fluorescence will make microarray assays more attractive and cost-effective.
The ampliPHOX colorimetric detection technology is intended for research applications, and has a limit of detection within one order of magnitude of traditional fluorescence11, with a main advantage being an approximate ten-fold lower instrument cost compared to the confocal microarray scanners required for fluorescence microarray detection. Another advantage is the compact size of the instrument which allows for portability and flexibility, unlike traditional fluorescence instruments. Because the polymerization technology is not as inherently linear as fluorescence detection, however, it is best suited for lower density microarray applications in which a yes/no answer for the presence of a certain sequence is desired, such as for pathogen detection arrays. Currently the maximum spot density compatible with ampliPHOX detection is &tilde;1800 spots/array. Because of the spot density limitations, higher density microarrays are not suitable for ampliPHOX detection.
Here, we present ampliPHOX colorimetric detection technology as a method of signal amplification on a low density microarray developed for the detection and characterization of influenza viruses (FluChip). Although this protocol uses the FluChip (a DNA microarray) as one specific application of ampliPHOX detection, any microarray incorporating biotinylated target can be labeled and detected in a similar manner. The microarray design and biotinylation of the target to be captured are the responsibility of the user. Once the biotinylated target has been captured on the array, ampliPHOX detection can be performed by first tagging the array with a streptavidin-label conjugate (ampliTAG). Upon light exposure using the ampliPHOX Reader instrument, polymerization of a monomer solution (ampliPHY) occurs only in regions containing ampliTAG-labeled targets. The polymer formed can be subsequently stained with a non-toxic solution to improve visual contrast, followed by imaging and analysis using a simple software package (ampliVIEW). The entire FluChip assay from un-extracted sample to result can be performed in about 6 hours, and the ampliPHOX detection steps described above can be completed in about 30 min.

ampliPHOX colorimetric detection technology is presented as an inexpensive alternative to fluorescence detection for microarrays. Based on photopolymerization, ampliPHOX produces solid polymer spots visible to the naked eye in just a few minutes. Results are then imaged and automatically interpreted with a simple yet powerful software package.

DNA microarrays have emerged as a powerful tool for pathogen detection.1-5  For instance, many examples of the ability to type and subtype influenza virus ...

Quantitative Analyses of all Influenza Type A Viral Hemagglutinins and Neuraminidases using Universal Antibodies in Simple Slot Blot Assays

Hemagglutinin (HA) and neuraminidase (NA) are two surface proteins of influenza viruses which are known to play important roles in the viral life cycle and the induction of protective immune responses1,2. As the main target for neutralizing antibodies, HA is currently used as the influenza vaccine potency marker and is measured by single radial immunodiffusion (SRID)3. However, the dependence of SRID on the availability of the corresponding subtype-specific antisera causes a minimum of 2-3 months delay for the release of every new vaccine. Moreover, despite evidence that NA also induces protective immunity4, the amount of NA in influenza vaccines is not yet standardized due to a lack of appropriate reagents or analytical method5. Thus, simple alternative methods capable of quantifying HA and NA antigens are desirable for rapid release and better quality control of influenza vaccines.
Universally conserved regions in all available influenza A HA and NA sequences were identified by bioinformatics analyses6-7. One sequence (designated as Uni-1) was identified in the only universally conserved epitope of HA, the fusion peptide6, while two conserved sequences were identified in neuraminidases, one close to the enzymatic active site (designated as HCA-2) and the other close to the N-terminus (designated as HCA-3)7. Peptides with these amino acid sequences were synthesized and used to immunize rabbits for the production of antibodies. The antibody against the Uni-1 epitope of HA was able to bind to 13 subtypes of influenza A HA (H1-H13) while the antibodies against the HCA-2 and HCA-3 regions of NA were capable of binding all 9 NA subtypes. All antibodies showed remarkable specificity against the viral sequences as evidenced by the observation that no cross-reactivity to allantoic proteins was detected. These universal antibodies were then used to develop slot blot assays to quantify HA and NA in influenza A vaccines without the need for specific antisera7,8. Vaccine samples were applied onto a PVDF membrane using a slot blot apparatus along with reference standards diluted to various concentrations. For the detection of HA, samples and standard were first diluted in Tris-buffered saline (TBS) containing 4M urea while for the measurement of NA they were diluted in TBS containing 0.01% Zwittergent as these conditions significantly improved the detection sensitivity. Following the detection of the HA and NA antigens by immunoblotting with their respective universal antibodies, signal intensities were quantified by densitometry. Amounts of HA and NA in the vaccines were then calculated using a standard curve established with the signal intensities of the various concentrations of the references used. 
Given that these antibodies bind to universal epitopes in HA or NA, interested investigators could use them as research tools in immunoassays other than the slot blot only.

A simple slot blot method was developed for the quantification of influenza viral hemagglutinin and neuraminidase using universal antibodies targeting their most conserved sequences identified through bioinformatics analyses. This innovative approach may provide a useful alternative to quantitative determination of all viral hemagglutinin and neuraminidase.

Hemagglutinin (HA) and neuraminidase (NA) are two surface proteins of influenza viruses which are known to play important roles in the viral life cycle ...

High-throughput Detection Method for Influenza Virus

Influenza virus is a respiratory pathogen that causes a high degree of morbidity and mortality every year in multiple parts of the world. Therefore, precise diagnosis of the infecting strain and rapid high-throughput screening of vast numbers of clinical samples is paramount to control the spread of pandemic infections. Current clinical diagnoses of influenza infections are based on serologic testing, polymerase chain reaction, direct specimen immunofluorescence and cell culture 1,2.
Here, we report the development of a novel diagnostic technique used to detect live influenza viruses. We used the mouse-adapted human A/PR/8/34 (PR8, H1N1) virus 3 to test the efficacy of this technique using MDCK cells 4. MDCK cells (104 or 5 x 103 per well) were cultured in 96- or 384-well plates, infected with PR8 and viral proteins were detected using anti-M2 followed by an IR dye-conjugated secondary antibody. M2 5 and hemagglutinin 1 are two major marker proteins used in many different diagnostic assays. Employing IR-dye-conjugated secondary antibodies minimized the autofluorescence associated with other fluorescent dyes. The use of anti-M2 antibody allowed us to use the antigen-specific fluorescence intensity as a direct metric of viral quantity. To enumerate the fluorescence intensity, we used the LI-COR Odyssey-based IR scanner. This system uses two channel laser-based IR detections to identify fluorophores and differentiate them from background noise. The first channel excites at 680 nm and emits at 700 nm to help quantify the background. The second channel detects fluorophores that excite at 780 nm and emit at 800 nm. Scanning of PR8-infected MDCK cells in the IR scanner indicated a viral titer-dependent bright fluorescence. A positive correlation of fluorescence intensity to virus titer starting from 102-105 PFU could be consistently observed. Minimal but detectable positivity consistently seen with 102-103 PFU PR8 viral titers demonstrated the high sensitivity of the near-IR dyes. The signal-to-noise ratio was determined by comparing the mock-infected or isotype antibody-treated MDCK cells.
Using the fluorescence intensities from 96- or 384-well plate formats, we constructed standard titration curves. In these calculations, the first variable is the viral titer while the second variable is the fluorescence intensity. Therefore, we used the exponential distribution to generate a curve-fit to determine the polynomial relationship between the viral titers and fluorescence intensities. Collectively, we conclude that IR dye-based protein detection system can help diagnose infecting viral strains and precisely enumerate the titer of the infecting pathogens.

This method describes the use of Infrared dye based imaging system for detection of H1N1 in bronchioalveolar lavage (BAL) fluid of infected mice at a high sensitivity. This methodology can be performed in a 96- or 384-well plate, requires &lt;10 &mu;l volume of test material and has the potential for concurrent screening of multiple pathogens.

Influenza virus is a respiratory pathogen that causes a high degree of morbidity and mortality every year in multiple parts of the world. Therefore, ...

Affinity Purification of Influenza Virus Ribonucleoprotein Complexes from the Chromatin of Infected Cells

Like all negative-strand RNA viruses, the genome of influenza viruses is packaged in the form of viral ribonucleoprotein complexes (vRNP), in which the single-stranded genome is encapsidated by the nucleoprotein (NP), and associated with the trimeric polymerase complex consisting of the PA, PB1, and PB2 subunits. However, in contrast to most RNA viruses, influenza viruses perform viral RNA synthesis in the nuclei of infected cells. Interestingly, viral mRNA synthesis uses cellular pre-mRNAs as primers, and it has been proposed that this process takes place on chromatin1. Interactions between the viral polymerase and the host RNA polymerase II, as well as between NP and host nucleosomes have also been characterized1,2.
Recently, the generation of recombinant influenza viruses encoding a One-Strep-Tag genetically fused to the C-terminus of the PB2 subunit of the viral polymerase (rWSN-PB2-Strep3) has been described. These recombinant viruses allow the purification of PB2-containing complexes, including vRNPs, from infected cells. To obtain purified vRNPs, cell cultures are infected, and vRNPs are affinity purified from lysates derived from these cells. However, the lysis procedures used to date have been based on one-step detergent lysis, which, despite the presence of a general nuclease, often extract chromatin-bound material only inefficiently. 
Our preliminary work suggested that a large portion of nuclear vRNPs were not extracted during traditional cell lysis, and therefore could not be affinity purified. To increase this extraction efficiency, and to separate chromatin-bound from non-chromatin-bound nuclear vRNPs, we adapted a step-wise subcellular extraction protocol to influenza virus-infected cells. Briefly, this procedure first separates the nuclei from the cell and then extracts soluble nuclear proteins (here termed the "nucleoplasmic" fraction). The remaining insoluble nuclear material is then digested with Benzonase, an unspecific DNA/RNA nuclease, followed by two salt extraction steps: first using 150 mM NaCl (termed "ch150"), then 500 mM NaCl ("ch500") (Fig. 1). These salt extraction steps were chosen based on our observation that 500 mM NaCl was sufficient to solubilize over 85% of nuclear vRNPs yet still allow binding of tagged vRNPs to the affinity matrix.
After subcellular fractionation of infected cells, it is possible to affinity purify PB2-tagged vRNPs from each individual fraction and analyze their protein and RNA components using Western Blot and primer extension, respectively. Recently, we utilized this method to discover that vRNP export complexes form during late points after infection on the chromatin fraction extracted with 500 mM NaCl (ch500)3.

Influenza viruses replicate their RNA genome in association with host-cell chromatin. Here, we present a method to purify intact viral ribonucleoprotein complexes from the chromatin of infected cells. Purified viral complexes can be analyzed by both Western blot and primer extension of protein and RNA content, respectively.

Like all negative-strand RNA viruses, the genome of influenza viruses is packaged in the form of viral ribonucleoprotein complexes (vRNP), in which the ...

Intranasal Administration of Recombinant Influenza Vaccines in Chimeric Mouse Models to Study Mucosal Immunity

Vaccines are one of the greatest achievements of mankind, and have saved millions of lives over the last century. Paradoxically, little is known about the physiological mechanisms that mediate immune responses to vaccines perhaps due to the overall success of vaccination, which has reduced interest into the molecular and physiological mechanisms of vaccine immunity. However, several important human pathogens including influenza virus still pose a challenge for vaccination, and may benefit from immune-based strategies. 
Although influenza reverse genetics has been successfully applied to the generation of live-attenuated influenza vaccines (LAIVs), the addition of molecular tools in vaccine preparations such as tracer components to follow up the kinetics of vaccination in vivo, has not been addressed. In addition, the recent generation of mouse models that allow specific depletion of leukocytes during kinetic studies has opened a window of opportunity to understand the basic immune mechanisms underlying vaccine-elicited protection. Here, we describe how the combination of reverse genetics and chimeric mouse models may help to provide new insights into how vaccines work at physiological and molecular levels, using as example a recombinant, cold-adapted, live-attenuated influenza vaccine (LAIV). We utilized laboratory-generated LAIVs harboring cell tracers as well as competitive bone marrow chimeras (BMCs) to determine the early kinetics of vaccine immunity and the main physiological mechanisms responsible for the initiation of vaccine-specific adaptive immunity. In addition, we show how this technique may facilitate gene function studies in single animals during immune responses to vaccines. We propose that this technique can be applied to improve current prophylactic strategies against pathogens for which urgent medical countermeasures are needed, for example influenza, HIV, Plasmodium, and hemorrhagic fever viruses such as Ebola virus.

There is an overall lack of knowledge about how vaccines work. Here we propose the combined use of reverse genetics and bone marrow chimeric mice to gain insight into the early host immune responses to vaccines with a special focus on dendritic cells and T cell immunity.

Vaccines are one of the greatest achievements of mankind, and have saved millions of lives over the last century. Paradoxically, little is known about the ...

Co-immunoprecipitation of the Mouse Mx1 Protein with the Influenza A Virus Nucleoprotein

Studying the interaction between proteins is key in understanding their function(s). A very powerful method that is frequently used to study interactions of proteins with other macromolecules in a complex sample is called co-immunoprecipitation. The described co-immunoprecipitation protocol allows to demonstrate and further investigate the interaction between the antiviral myxovirus resistance protein 1 (Mx1) and one of its viral targets, the influenza A virus nucleoprotein (NP). The protocol starts with transfected mammalian cells, but it is also possible to use influenza A virus infected cells as starting material. After cell lysis, the viral NP protein is pulled-down with a specific antibody and the resulting immune-complexes are precipitated with protein G beads. The successful pull-down of NP and the co-immunoprecipitation of the antiviral Mx1 protein are subsequently revealed by western blotting. A prerequisite for successful co-immunoprecipitation of Mx1 with NP is the presence of N-ethylmaleimide (NEM) in the cell lysis buffer. NEM alkylates free thiol groups. Presumably this reaction stabilizes the weak and/or transient NP&#8211;Mx1 interaction by preserving a specific conformation of Mx1, its viral target or an unknown third component. An important limitation of co-immunoprecipitation experiments is the inadvertent pull-down of contaminating proteins, caused by nonspecific binding of proteins to the protein G beads or antibodies. Therefore, it is very important to include control settings to exclude false positive results. The described co-immunoprecipitation protocol can be used to study the interaction of Mx proteins from different vertebrate species with viral proteins, any pair of proteins, or of a protein with other macromolecules. The beneficial role of NEM to stabilize weak and/or transient interactions needs to be tested for each interaction pair individually.

This co-immunoprecipitation protocol allows to study the interaction between the influenza A virus nucleoprotein and the antiviral Mx1 protein in human cells. The protocol emphasizes the importance of N-ethylmaleimide for successful co-immunoprecipitation of Mx1 and influenza A virus nucleoprotein.

Studying the interaction between proteins is key in understanding their function(s). A very powerful method that is frequently used to study interactions ...

Nasal Wipes for Influenza A Virus Detection and Isolation from Swine

Surveillance for influenza A viruses in swine is critical to human and animal health because influenza A virus rapidly evolves in swine populations and new strains are continually emerging. Swine are able to be infected by diverse lineages of influenza A virus making them important hosts for the emergence and maintenance of novel influenza A virus strains. Sampling pigs in diverse settings such as commercial swine farms, agricultural fairs, and live animal markets is important to provide a comprehensive view of currently circulating IAV strains. The current gold-standard ante-mortem sampling technique (i.e. collection of nasal swabs) is labor intensive because it requires physical restraint of the pigs. Nasal wipes involve rubbing a piece of fabric across the snout of the pig with minimal to no restraint of the animal. The nasal wipe procedure is simple to perform and does not require personnel with professional veterinary or animal handling training. While slightly less sensitive than nasal swabs, virus detection and isolation rates are adequate to make nasal wipes a viable alternative for sampling individual pigs when low stress sampling methods are required. The proceeding protocol outlines the steps needed to collect a viable nasal wipe from an individual pig.

The authors present a protocol to collect swine nasal wipes to detect and isolate influenza A viruses.

Surveillance for influenza A viruses in swine is critical to human and animal health because influenza A virus rapidly evolves in swine populations and ...

A Miniaturized Glycan Microarray Assay for Assessing Avidity and Specificity of Influenza A Virus Hemagglutinins

Influenza A virus (IAV) hemagglutinins recognize sialic acids on the cell surface as functional receptors to gain entry into cells. Wild waterfowl are the natural reservoir for IAV, but IAV can cross the species barrier to poultry, swine, horses and humans. Avian viruses recognize sialic acid attached to a penultimate galactose by a &#945;2-3 linkage (avian-type receptors) whereas human viruses preferentially recognize sialic acid with a &#945;2-6 linkage (human-type receptors). To monitor if avian viruses are adapting to human type receptors, several methods can be used. Glycan microarrays with diverse libraries of synthetic sialosides are increasingly used to evaluate receptor specificity. However, this technique is not used for measuring avidities. Measurement of avidity is typically achieved by evaluating the binding of serially diluted hemagglutinin or virus to glycans adsorbed to conventional polypropylene 96-well plates. In this assay, glycans with &#945;2-3 or &#945;2-6 sialic acids are coupled to biotin and adsorbed to streptavidin plates, or are coupled to polyacrylamide (PAA) which directly adsorb to the plastic. We have significantly miniaturized this assay by directly printing PAA-linked sialosides and their non PAA-linked counterparts on micro-well glass slides. This set-up, with 48 arrays on a single slide, enables simultaneous assays of 6 glycan binding proteins at 8 dilutions, interrogating 6 different glycans, including two non-sialylated controls. This is equivalent to 18x 96-well plates in the traditional plate assay. The glycan array format decreases consumption of compounds and biologicals and thus greatly enhances efficiency.

Using a printed glycan microarray strategy, a conventional 96-well plate assay was miniaturized for analysis of influenza A virus hemagglutinin avidity and specificity for sialic acid containing receptors.

Influenza A virus (IAV) hemagglutinins recognize sialic acids on the cell surface as functional receptors to gain entry into cells. Wild waterfowl are the ...

In Vitro Disassembly of Influenza A Virus Capsids by Gradient Centrifugation

Acid-triggered molecular processes closely control cell entry of many viruses that enter through the endocytic system. In the case of influenza A virus (IAV), virus fusion with the endosomal membrane as well as the subsequent disassembly of the viral capsid, called uncoating, is governed by the ionic conditions inside endocytic vesicles. The early steps in the virus life cycle are hard to study because endosomes cannot be directly accessed experimentally, creating the need for an in vitro approach. Here, we describe a method based on velocity gradient centrifugation of purified virions through a two-layer glycerol gradient, which enables analysis of the IAV core and its stability. The gradient contains a non-ionic detergent (NP-40) in its lower layer to remove the viral membrane by solubilization as the virus sediments toward the bottom. At neutral pH, viral cores are pelleted as stable structures. The major core components, matrix protein (M1) and the viral ribonucleoproteins (vRNPs), can be clearly identified in the pellet fraction by SDS-PAGE. Decreasing the pH to 6.0 or lower in the bottom layer selectively removes M1 from the pellet followed by release of vRNPs at more acidic conditions. Viral protein bands on Coomassie-stained gels can be subjected to densitometric quantification to monitor intermediate states of IAV disassembly. Besides pH, other factors that influence viral core stability can be assessed, such as salt concentration and putative viral uncoating factors, simply by modifying the detergent-containing glycerol layer accordingly. Taken together, the presented technique allows highly reproducible and quantitative analysis of viral uncoating in vitro. It can be applied to other enveloped viruses that undergo complex uncoating processes.

In Vitro Disassembly of Influenza A Virus Capsids by Gradient Centrifugation

Disassembly of influenza A virus cores during virus entry into host cells is a multistep process. We describe an in vitro method to analyze the early stages of viral uncoating. In this approach, velocity gradient centrifugation is used to biochemically dissect the steps that initiate uncoating under defined conditions.

Acid-triggered molecular processes closely control cell entry of many viruses that enter through the endocytic system. In the case of influenza A virus ...