We would like to thank the Deutsche Forschungsgemeinschaft (DFG) grant BE1911/8-1, the LOEWE project GLUE, and the collaborative research center Transport and Communication across Membranes (SFB807) for financial support.

1. Preparation of S30 lysate
<ol>
	<li>Day 1: Streak out cells from glycerol stocks on an LB agar plate and incubate at 37 &#176;C overnight.</li>
	<li>Day 2: Inoculate 200 mL of LB medium with the cells from the agar plate and incubate at 37 &#176;C for 12-14 h.</li>
	<li>Day 3: Inoculate 10 L of sterile YPTG medium (10 g/L yeast extract, 16 g/L tryptone, 5 g/L NaCl, 19.8 g/L glucose, 4.4 mM KH2PO4, 8 mM K2HPO4) tempered to ...

<ol>
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K., 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=Reconstitution+of+membrane+proteins+in+phospholipid+bilayer+nanodiscs.">Reconstitution of membrane proteins in phospholipid bilayer nanodiscs.</a> Methods in Enzymology. 464 (09), 211-231 (2009).</li><li>Frauenfeld, J., 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=A+novel+lipoprotein+nanoparticle+system+for+membrane+proteins.">A novel lipoprotein nanoparticle system for membrane proteins.</a> Nature Methods. 13 (4), 345-351 (2016).</li><li>Yang, Z., 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=Membrane+protein+stability+can+be+compromised+by+detergent+interactions+with+the+extramembranous+soluble+domains.">Membrane protein stability can be compromised by detergent interactions with the extramembranous soluble domains.</a> Protein Science. 23 (6), 769-789 (2014).</li><li>Seddon, A. 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The authors have nothing to disclose.

The setup and strategies to optimize the CF expression and co-translational insertion of functional MPs into nanodiscs are described. The required equipment comprises a bioreactor, a French press device or similar, an UV/VIS and fluorescence reader, CF reaction vessels suitable for a two-compartment configuration setup, and a temperature-controlled incubator. Further standard equipment are centrifuges for harvesting E. coli cells as well as tabletop centrifuges reaching at least 30,000 x g for preparati...

Membrane proteins (MPs) are challenging targets in protein production studies due to their insolubility in aqueous environments. Conventional MP production platforms comprise cell-based systems such as E. coli, yeast, or eukaryotic cells. The synthesized recombinant MPs are either extracted from cell membranes or refolded from inclusion bodies1. After detergent solubilization, MPs can be transferred into suitable membrane environments by established in vitro reconstitution protocols. Besides vesicles and liposomes, MP reconstitution into planar membranes in the form of nanodiscs2 or salipro3 particles have become routine techniques in recent times. However, all these strategies imply detergent contact with MPs that can result in destabilization, dissociation of oligomers, and even loss of protein structure and activity4. Screening for optimal detergent solubilization and reconstitution conditions can therefore be tedious and time consuming5.
The open nature of cell-free (CF) systems allows the expression reaction to be directly supplied with preformed membranes with a defined lipid composition. In this lipid-based expression mode (L-CF), the synthesized MPs have the opportunity to co-translationally insert into the provided bilayers6,7 (Figure 1). Nanodiscs consisting of a membrane scaffold protein (MSP) surrounding a planar lipid bilayer disc8 appear to be particularly suitable for this strategy9,10. Nanodiscs can routinely be assembled in vitro with a variety of different lipids, they are very stable, and stocks can be concentrated up to 1 mM. However, MSP expression in E. coli and its purification is necessary. As an alternative strategy, MSP can be co-expressed together with the target MP in CF reactions supplied with liposomes11,12,13. DNA templates for both MSP and MP are added into the reaction and MP/nanodiscs can form upon expression. While MSP production is avoided, the co-expression strategy requires careful fine-tuning of the final DNA template concentrations and higher variations in the efficiency of sample production can be expected.
The co-translational insertion of MPs into membranes of preformed nanodiscs is a non-physiological and still largely unknown mechanism independent from translocon machineries and signal sequences13,14,15,16. Major determinants of the insertion efficiency are the type of membrane protein as well as the lipid composition of the provided membrane, with a frequent preference for negatively charged lipids15,17. As the nanodisc membranes are relatively confined in size, a substantial amount of lipids is released upon MP insertion18. Variation of nanodisc size enables insertion and tuning of higher oligomeric MP complexes15,18. Among others, the correct assembly of homooligomeric complexes was shown for the ion channel KcsA, for the ion pump proteorhodopsin (PR) and for the multidrug transporter EmrE15,18. MPs are likely to enter the symmetric nanodisc membrane from both sides at relatively equal frequency. It should therefore be considered that different monomers or oligomers inserted into one nanodisc may have opposite orientations. However, a bias in orientation could be caused by cooperative insertion mechanisms as reported for the formation of PR hexamers and KcsA tetramers18. A further bias in MP orientation might result from orientation switches of inserted MPs probably at the rim of the nanodisc membranes.
The production of CF lysates from E. coli strains is a reliable routine technique and can be performed in almost any biochemical laboratory19,20. It should be considered that besides disulfide bridge formation, most other post-translational modifications are absent if a MP is synthesized using E. coli lysates. While this might generate more homogenous samples for structural studies, it may be necessary to exclude potential effects on the function of individual MP targets. However, the efficient production of high quality samples of G-protein coupled receptors (GPCR)14,21,22, eukaryotic transporters23 or large heteromeric assemblies24 in E. coli CF lysates indicates their suitability for even complex targets. Preparative scale amounts (&#8776; 1 mg/mL) of a sample can be obtained with the two-compartment continuous exchange cell-free (CECF) configuration, composed of a reaction mixture (RM) and a feeding mixture (FM) compartment. The FM volume exceeds the RM volume 15 to 20-fold and provides a reservoir of low-molecular weight energy compounds and precursors19. The expression reaction is thus extended for several hours and the final yield of the MP target is increased. The RM and FM compartments are separated by a dialysis membrane with a 10-14 kDa cutoff. The two compartments require a special design of the CECF reaction container (Figure 1). Commercial dialysis cassettes as RM containers in combination with tailored plexiglass containers holding the FM are suitable examples. MP yields can further be manipulated by modifying the RM:FM ratios or by exchanging the FM after a certain period of incubation.
Yield and quality of a MP frequently require intense optimization of reaction parameters. An important advantage of CF expression is the possibility to modify and fine tune almost any compound according to the individual requirements of a MP. A straightforward strategy is to focus first on improving the yield of a MP by establishing a basic production protocol and then to optimize MP quality by fine tuning reaction and folding conditions. The absence of physiological processes in CF lysates and their reduced complexity result in high success rates for the efficient production of MPs25. Routine considerations for DNA template design and optimization of Mg2+ ion concentration are in most cases sufficient to obtain satisfactory yields26. Depending on expression mode, optimization of MP quality can become time consuming, as a larger variety of parameters need to be screened14,17,22.
To establish the described CF expression platform, protocols are necessary for the production of E. coli CF lysate (i), T7 RNA polymerase (ii), nanodiscs (iii), and the basic CECF reaction compounds (iv) (Figure 1). The E. coli K12 strain A1927 or BL21 derivatives are frequently used for the preparation of efficient S30 (centrifugation at 30,000 x g) lysates. Besides S30 lysates, corresponding lysates centrifuged at other g-forces (e.g. S12) may be used. The lysates differ in expression efficiency and in proteome complexity. The proteome of the S30 lysate prepared by the described protocol has been studied in detail28. The S30 proteome still contains some residual outer membrane proteins which could give background problems upon expression and analysis of ion channels. For such targets, the use of S80-S100 lysates is recommended. A valuable modification during lysate preparation is the induction of the SOS response by combined heat shock and ethanol supply at mid-log growth phase of the cells. The resulting HS30 lysates are enriched in chaperones and can be used in blends with S30 lysates for improved MP folding22. CF expression in E. coli lysates is operated as a coupled transcription/translation process with DNA templates containing promoters controlled by T7 RNA polymerase (T7RNAP). The enzyme can be efficiently produced in BL21(DE3) Star cells carrying the pAR1219 plasmid29.
Two copies of MSP1E3D1 assemble into one nanodisc with a diameter of 10-12 nm, but the protocol described below may also work for other MSP derivatives. However, nanodiscs larger than those formed with MSP1E3D1 tend to be less stable while smaller nanodiscs formed with MSP derivatives such as MSP1 may not accommodate larger MPs or MP complexes. MSP1E3D1 nanodiscs can be assembled in vitro with a large variety of different lipids or lipid mixtures. Preformed nanodiscs are usually stable for &#62; 1 year at -80 &#176;C, while stability may vary for different lipid components. For the screening of lipid effects on folding and stability of a MP, it is necessary to prepare a comprehensive set of nanodiscs assembled with a representative variety of lipids/lipid mixtures. The following lipids may give a good starting selection: 1,2-Dimyristoyl-sn-glycero-3-phospho-(1&#39;-rac-glycerol) (DMPG), 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), 1,2 dioleoyl-sn-glycero-3-phospho-rac-(1-glycerol) (DOPG), 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1&#39;-rac-glycerol) (POPG) and 1-palmitoyl-2-oleoyl-glycero-3-phosphocholine (POPC).
A protocol for the preparation of a 3 mL CECF reaction is described. Further up or down scaling in a 1:1 ratio is possible. For the two-compartment CECF configuration, a RM containing all compounds and a FM containing only the low-molecular weight compounds have to be prepared. Commercial 3 mL dialysis devices with 10-14 kDa MWCO can be used as RM containers, which are then placed into custom made plexiglass containers holding the FM (Figure 1D)30. The ratio of RM:FM is 1:20, so 60 mL of FM have to be prepared for 3 mL RM. Quality, concentration, or type of several components can be critical for the final yield and/or quality of the synthesized MP (Table 1). DNA templates should be prepared according to published guidelines and codon optimization of the reading frame of the target can further significantly improve product yield26. For preparative scale CECF reaction, an established protocol for the production of PR is described. To establish expression protocols for new MPs, it is usually necessary to perform optimization screens of certain compounds (Table 1) to improve yield and quality. For Mg2+ ions, a well-focused concentration optimum does exist that frequently shows significant variation for different DNA templates. Other CF reaction compounds such as new batches of T7RNAP or S30 lysates may further shift the optimal Mg2+ concentration. As an example, a typical Mg2+ screen within the range of 14-24 mM concentration and in steps of 2 mM is described. Each concentration is screened in duplicates and in analytical scale CECF reactions. As CECF reaction container, custom-made Mini-CECF Plexiglas containers30 holding the RM are used in combination with standard 24-well microplates holding the FM (Figure 1B). Alternatively, commercial dialysis cartridges in combination with 96-deep well microplates or other commercial dialyzer devices in appropriate setups may be used (Figure 1C).

<table>
	<tbody>
		<tr>
			<td>1,2-dimyristoyl-sn-glycero-3-phospho-(1&#39;-rac-glycerol) (sodium salt) (DMPG)</td>
			<td>Avanti Polar Lipids (USA)</td>
			<td>840445P</td>
			<td></td>
		</tr>
		<tr>
			<td>1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC)</td>
			<td>Avanti Polar Lipids (USA)</td>
			<td>850345C</td>
			<td></td>
		</tr>
		<tr>
			<td>1,2-dioleoyl-sn-glycero-3-phosphocholine (sodium salt) (DOPC)</td>
			<td>Avanti Polar Lipids (USA)</td>
			<td>850375C</td>
			<td></td>
		</tr>
		<tr>
			<td>1,2 dioleoyl-sn-glycero-3-phospho-rac-(1-glycerol) (sodium salt) (DOPG)</td>
			<td>Avanti Polar Lipids (USA)</td>
			<td>840475C</td>
			<td></td>
		</tr>
		<tr>
			<td>1-palmitoyl-2-oleoyl-glycero-3-phosphocholine (POPC)</td>
			<td>Avanti Polar Lipids (USA)</td>
			<td>850457C</td>
			<td></td>
		</tr>
		<tr>
			<td>1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1&#39;-rac-glycerol) (sodium salt) (POPG)</td>
			<td>Avanti Polar Lipids (USA)</td>
			<td>840034C</td>
			<td></td>
		</tr>
		<tr>
			<td>2-Amino-2-(hydroxymethyl)-propan-1,3-diol (Tris)</td>
			<td>Carl Roth (Germany)</td>
			<td>4855</td>
			<td></td>
		</tr>
		<tr>
			<td>2-Mercaptoethanol</td>
			<td>Carl Roth (Germany)</td>
			<td>4227</td>
			<td></td>
		</tr>
		<tr>
			<td>2-Propanol</td>
			<td>Carl Roth (Germany)</td>
			<td>9781</td>
			<td></td>
		</tr>
		<tr>
			<td>[3H]-dihydroalprenolol Hydrochloride</td>
			<td>American Radiolabeled Chemicals (USA)</td>
			<td>ART0134</td>
			<td></td>
		</tr>
		<tr>
			<td>Acetyl phosphate lithium potassium salt (ACP)</td>
			<td>Merck (Germany)</td>
			<td>1409</td>
			<td></td>
		</tr>
		<tr>
			<td>Adenosine 5&#8217;-triphosphate (ATP)</td>
			<td>Sigma Aldrich (Germany)</td>
			<td>A9251</td>
			<td></td>
		</tr>
		<tr>
			<td>Alprenolol hydrochloride</td>
			<td>Merck (Germany)</td>
			<td>A0360000</td>
			<td></td>
		</tr>
		<tr>
			<td>Anion exchange chromatography column material: Q-sepharose&#174;</td>
			<td>Sigma-Aldrich (Germany)</td>
			<td>Q1126</td>
			<td></td>
		</tr>
		<tr>
			<td>Autoclave Type GE 446EC-1</td>
			<td>Gettinge (Germany)</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Bioreactor Type 884 124/1</td>
			<td>B.Braun (Germany)</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Centrifuge</td>
			<td>Sorvall RC12BP+; Thermo Scientific (Germany); Sorvall RC-5C; Thermo Scientific (Germany); Mikro 22 R; Hettich (Germany)</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Cholic acid</td>
			<td>Carl Roth (Germany)</td>
			<td>8137</td>
			<td></td>
		</tr>
		<tr>
			<td>Coomassie Brilliant Blue R250</td>
			<td>Carl Roth (Germany)</td>
			<td>3862</td>
			<td></td>
		</tr>
		<tr>
			<td>Culture flasks 500 ml baffled flasks, 2 l baffled flasks</td>
			<td>Schott Duran (Germany)</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Cytidine 5&#39;-triphosphate disodium salt</td>
			<td>Sigma-Aldrich (Germany)</td>
			<td>C1506</td>
			<td></td>
		</tr>
		<tr>
			<td>D-glucose monohydrate</td>
			<td>Carl Roth (Germany)</td>
			<td>6780</td>
			<td></td>
		</tr>
		<tr>
			<td>Di-potassiumhydrogen phosphate trihydrate</td>
			<td>Carl Roth (Germany)</td>
			<td>6878</td>
			<td></td>
		</tr>
		<tr>
			<td>Dialysis tubing SpectrumTM Labs Spectra/PorTM 12-14 kD MWCO Standard RC tubing</td>
			<td>Fisher Scientific (Germany)</td>
			<td>8700152</td>
			<td></td>
		</tr>
		<tr>
			<td>Dithiothreit</td>
			<td>Carl Roth (Germany)</td>
			<td>6908</td>
			<td></td>
		</tr>
		<tr>
			<td>Ethanol</td>
			<td>Carl Roth (Germany)</td>
			<td>K928</td>
			<td></td>
		</tr>
		<tr>
			<td>Folinic acid calcium salt hydrate</td>
			<td>Sigma-Aldrich (Germany)</td>
			<td>47612</td>
			<td></td>
		</tr>
		<tr>
			<td>French pressure cell disruptor</td>
			<td>SLM; Amico Instruments (USA)</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Glycerol</td>
			<td>Carl Roth (Germany)</td>
			<td>3783</td>
			<td></td>
		</tr>
		<tr>
			<td>Guanosine 5&#39;-triphosphate di-sodium salt (GTP)</td>
			<td>Sigma-Aldrich (Germany)</td>
			<td>G8877</td>
			<td></td>
		</tr>
		<tr>
			<td>Hydrochloric Acid</td>
			<td>Carl Roth (Germany)</td>
			<td>K025</td>
			<td></td>
		</tr>
		<tr>
			<td>IMAC column: HiTrap IMAC HP 5 mL</td>
			<td>GE Life Sciences (Germany)</td>
			<td>GE17-5248</td>
			<td></td>
		</tr>
		<tr>
			<td>Imidazole</td>
			<td>Carl Roth (Germany)</td>
			<td>3899</td>
			<td></td>
		</tr>
		<tr>
			<td>Isopropyl-&#946;-D-thiogalactopyranosid (IPTG)</td>
			<td>Carl Roth (Germany)</td>
			<td>2316</td>
			<td></td>
		</tr>
		<tr>
			<td>Kanamycin</td>
			<td>Carl Roth (Germany)</td>
			<td>T832</td>
			<td></td>
		</tr>
		<tr>
			<td>L-Alanine</td>
			<td>Carl Roth (Germany)</td>
			<td>3076.1</td>
			<td></td>
		</tr>
		<tr>
			<td>L-Arginine</td>
			<td>Carl Roth (Germany)</td>
			<td>6908</td>
			<td></td>
		</tr>
		<tr>
			<td>L-Asparagine</td>
			<td>Carl Roth (Germany)</td>
			<td>HN23</td>
			<td></td>
		</tr>
		<tr>
			<td>L-Aspartic Acid</td>
			<td>Carl Roth (Germany)</td>
			<td>T202</td>
			<td></td>
		</tr>
		<tr>
			<td>L-Cysteine</td>
			<td>Carl Roth (Germany)</td>
			<td>T203</td>
			<td></td>
		</tr>
		<tr>
			<td>L-Glutamic Acid</td>
			<td>Carl Roth (Germany)</td>
			<td>3774</td>
			<td></td>
		</tr>
		<tr>
			<td>L-Glutamine</td>
			<td>Carl Roth (Germany)</td>
			<td>3772</td>
			<td></td>
		</tr>
		<tr>
			<td>L-Glycine</td>
			<td>Carl Roth (Germany)</td>
			<td>3187</td>
			<td></td>
		</tr>
		<tr>
			<td>L-Histidine</td>
			<td>Carl Roth (Germany)</td>
			<td>3852</td>
			<td></td>
		</tr>
		<tr>
			<td>L-Isoleucine</td>
			<td>Carl Roth (Germany)</td>
			<td>3922</td>
			<td></td>
		</tr>
		<tr>
			<td>L-Leucine</td>
			<td>Carl Roth (Germany)</td>
			<td>1699</td>
			<td></td>
		</tr>
		<tr>
			<td>L-Lysine</td>
			<td>Carl Roth (Germany)</td>
			<td>4207</td>
			<td></td>
		</tr>
		<tr>
			<td>L-Methionine</td>
			<td>Carl Roth (Germany)</td>
			<td>9359</td>
			<td></td>
		</tr>
		<tr>
			<td>L-Proline</td>
			<td>Carl Roth (Germany)</td>
			<td>1713</td>
			<td></td>
		</tr>
		<tr>
			<td>L-Phenylalanine</td>
			<td>Carl Roth (Germany)</td>
			<td>1709</td>
			<td></td>
		</tr>
		<tr>
			<td>L-Serine</td>
			<td>Carl Roth (Germany)</td>
			<td>4682</td>
			<td></td>
		</tr>
		<tr>
			<td>L-Threonine</td>
			<td>Carl Roth (Germany)</td>
			<td>1738</td>
			<td></td>
		</tr>
		<tr>
			<td>L-Tryptophane</td>
			<td>Carl Roth (Germany)</td>
			<td>7700</td>
			<td></td>
		</tr>
		<tr>
			<td>L-Tyrosine</td>
			<td>Carl Roth (Germany)</td>
			<td>T207</td>
			<td></td>
		</tr>
		<tr>
			<td>MD100 dialysis units</td>
			<td>Scienova (Germany)</td>
			<td>40077</td>
			<td></td>
		</tr>
		<tr>
			<td>N-2-Hydroxyethylpiperazine-N&#39;-2-ethansulfonic acid (HEPES)</td>
			<td>Carl Roth (Germany)</td>
			<td>6763</td>
			<td></td>
		</tr>
		<tr>
			<td>n-dodecylphosphocholine</td>
			<td>Antrace (USA)</td>
			<td>F308S</td>
			<td></td>
		</tr>
		<tr>
			<td>PAGE chamber: Mini-Protean Tetra Cell</td>
			<td>Biorad (Germany)</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>PAGE gel casting system: Mini Protean Handcast systems</td>
			<td>Biorad (Germany)</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>PAGE gel power supply: Power Pac 3000</td>
			<td>Biorad (Germany)</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Tryptone/peptone from caseine</td>
			<td>Carl Roth (Germany)</td>
			<td>6681</td>
			<td></td>
		</tr>
		<tr>
			<td>Peristaltic pump: ip-12</td>
			<td>Ismatec (Germany)</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Phosphoenol pyruvate monopotassium salt</td>
			<td>Sigma Aldrich (Germany)</td>
			<td>860077</td>
			<td></td>
		</tr>
		<tr>
			<td>Potassium dihydrogen phosphate</td>
			<td>Carl Roth (Germany)</td>
			<td>P018</td>
			<td></td>
		</tr>
		<tr>
			<td>Potassium acetate</td>
			<td>Carl Roth (Germany)</td>
			<td>4986</td>
			<td></td>
		</tr>
		<tr>
			<td>Potassium chloride</td>
			<td>Carl Roth (Germany)</td>
			<td>6781</td>
			<td></td>
		</tr>
		<tr>
			<td>Pyruvate Kinase</td>
			<td>Roche (Germany)</td>
			<td>10109045001</td>
			<td></td>
		</tr>
		<tr>
			<td>Scintillation counter: Hidex 300 SL</td>
			<td>Hidex (Finland)</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>SDS pellets</td>
			<td>Carl Roth (Germany)</td>
			<td>8029</td>
			<td></td>
		</tr>
		<tr>
			<td>Sodium azide</td>
			<td>Sigma-Aldrich (Germany)</td>
			<td>K305</td>
			<td></td>
		</tr>
		<tr>
			<td>Sodium chloride</td>
			<td>Carl Roth (Germany)</td>
			<td>P029</td>
			<td></td>
		</tr>
		<tr>
			<td>Spectrophotometer Nanodrop</td>
			<td>Peqlab (Germany)</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Spectrophotometer/fluorescence reader Spark&#174;</td>
			<td>Tecan (Switzerland)</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>tRNA (E. coli)</td>
			<td>Roche (Germany)</td>
			<td>10109550001</td>
			<td></td>
		</tr>
		<tr>
			<td>Ultra sonificator</td>
			<td>Labsonic U, B. Braun (Germany)</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Uridine 5&#8217;-triphosphate tri-sodium salt (UTP)</td>
			<td>Sigma-Aldrich (Germany)</td>
			<td>U6625</td>
			<td></td>
		</tr>
		<tr>
			<td>Y-30 antifoam</td>
			<td>Sigma-Aldrich (Germany)</td>
			<td>A6457</td>
			<td></td>
		</tr>
		<tr>
			<td>Yeast extract</td>
			<td>Carl Roth (Germany)</td>
			<td>2904</td>
			<td></td>
		</tr>
	</tbody>
</table>

co-translational insertion of membrane proteins into preformed nanodiscs

Cell-free expression systems allow the tailored design of reaction environments to support the functional folding of even complex proteins such as membrane proteins. The experimental procedures for the co-translational insertion and folding of membrane proteins into preformed and defined membranes supplied as nanodiscs are demonstrated. The protocol is completely detergent-free and can generate milligrams of purified samples within one day. The resulting membrane protein/nanodisc samples can be used for a variety of functional studies and structural applications such as crystallization, nuclear magnetic resonance, or electron microscopy. The preparation of basic key components such as cell-free lysates, nanodiscs with designed membranes, critical stock solutions as well as the assembly of two-compartment cell-free expression reactions is described. Since folding requirements of membrane proteins can be highly diverse, a major focus of this protocol is the modulation of parameters and reaction steps important for sample quality such as critical basic reaction compounds, membrane composition of nanodiscs, redox and chaperone environment, or DNA template design. The whole process is demonstrated with the synthesis of proteorhodopsin and a G-protein coupled receptor.

The impact of fine-tuning reaction compounds on the final yield or quality of synthesized MPs is exemplified. A frequent routine approach is to adjust the optimal Mg2+ concentration in CF reactions by expression of green fluorescent protein (GFP) as a convenient monitor of system efficiency. As an example, GFP was synthesized from a pET-21a(+) vector at Mg2+ concentrations between 14 and 24 mM (Figure 2). SDS-PAGE analysis identified the optimal Mg2+ concentr...

Watch this Scientific Journal Video about Co-Translational Insertion of Membrane Proteins into Preformed Nanodiscs at JoVE.com

Co-Translational Insertion of Membrane Proteins into Preformed Nanodiscs

Co-translational insertion into pre-formed nanodiscs makes it possible to study cell-free synthesized membrane proteins in defined lipid environments without contact with detergents. This protocol describes the preparation of essential system components and the critical parameters for improving expression efficiency and sample quality.

Cell-free expression systems allow the tailored design of reaction environments to support the functional folding of even complex proteins such as ...

The protocol is highly efficient and versatile. It can be applied to almost any membrane protein, and the whole production procedure is completed within a day. The synthesized membrane proteins can be stabilized already co-translationally by supplied ligands.Detergent contacts are completely avoided, and the proteins insert and fold directly in provided and tailored membrane environments. Structural approaches, functional characterization, and antibody generation are prime areas of application. The technique is moreover straightforward in analyzing lipid affects on membrane protein folding and function.On day three of culture, inoculate 10 liters of sterile 37 degree Celsius YPTG medium in a 15-liter stirred-tank reactor with 100 milliliters of pre-culture. And cultivate the bacteria at 37 degrees Celsius with 500 revolutions per minute in high aeration. To prevent excessive foaming, add sterile anti-foam.When the cells reach the mid-log phase, add 300 milliliters of ethanol to the culture and incubate the cells at 42 degrees Celsius with 500 rotations per minute in high aeration for another 45 minutes. At the end of the incubation, harvest the cells by centrifugation, and use a spatula and a pipette to resuspend the cells completely in 300 milliliters of S30A buffer. Centrifuge the resuspended bacteria two times, washing the pellet with 300 milliliters of fresh S30A per wash.After the second wash, resuspend the cells in a 1.1 volume of S30B buffer. Load the suspension into a pre-cooled French-press pressure cell and disrupt cells at 1, 000 pounds per square inch gauge one time. To remove any unstable components and to release the mRNA from the ribosomes, adjust the lysate to 0.4 molar sodium chloride and incubate the sample for 45 minutes in a 42-degree Celsius water bath.To assemble the membrane scaffold protein nanodiscs, mix the MSP1E3D1 solution with the lipid solution and add DPC to a final concentration of 0.1%Adjust the reaction to the appropriate final volume according to the table and incubate the reactions for one hour at 21 degrees Celsius under gentle agitation. At the end of the incubation, pre-wet the membrane of a dialysis cassette with disc-formation buffer and use a syringe to load the assembly mixture into the cassette. Next dialyze the reaction against three five-liter volumes of disc-formation buffer for 10 to 20 hours at room temperature with stirring before transferring the mixture into disc formation-buffer equilibrated centrifugal filter units with a 10 kilodalton molecular weight cutoff.Then concentrate the reaction to at least 300 micromolar by centrifugation using a UV-Vis reader to measure the concentration. To prepare a three-milliliter continuous exchange cell-free reaction, equilibrate the membrane of a three-milliliter dialysis cassette in 100 millimolar tris-acetate. And use a syringe to transfer the reaction mixture into the cassette.Using a syringe, remove excess air from the reaction mixture compartment and place the cassette into the dialysis chamber. Fill the chamber with 60 milliliters of feeding mixture and place the lid on the chamber. After fixing the lid into place, incubate the chamber for 12 to 16 hours at 30 degrees Celsius in 200 revolutions per minute.To prepare a 60-microliter cell exchange cell-free reaction setup for magnesium ion screening, add 60 microliters of our freshly prepared 3X Master Mix to the reaction mixture and 825 microliters to the feeding mixture. And fill the feeding mixture with water. After vortexing, add 825 microliters of feeding mixture to each of three wells of a 24-well microplate.Cut regenerated cellulose dialysis tubes with a 10 to 14 kilodalton molecular weight cutoff into approximately 25-by-20 millimeter-sized pieces, and store the pieces in water supplemented with 0.05%sodium azide. Before mini-cell exchange cell-free container assembly, use a tissue to remove any excess water from the dialysis membranes, and place one membrane piece on each container. Fix the membranes with a polytetrafluoroethylene ring and transfer the mini container into one well of feeding mixture.Add the high-molecular components to the reaction mixture, as indicated in the table, and mix by pipetting. Add 60 microliters of reaction mixture to the reaction container, taking care to avoid air bubbles. And wrap two eight-by-10 centimeter sheets of a sealing thermoplastic film around the 24-well plate.Fix the lid of the well plate in place with tape and place the plate at 30 degrees Celsius in 200 revolutions per minute with agitation for 12 to 16 hours. The next morning, use a pipette tip to pierce through the dialysis membrane at the mini-cell exchange cell-free container and aspirate the reaction mixture. Transfer the reaction mixture into new centrifuge tubes and centrifuge to remove precipitates.Then transfer the supernatant into a new tube. Here a representative impact of fine-tuning reaction compounds on the final yield or quality of synthesized membrane proteins is shown. In this analysis, the overall synthesis and nanodisc solubilization of the T-beta-one adrenergic receptor was similar under all of the analyzed membrane compositions.In contrast, a much higher variation was detected in the quality of the synthesized G protein-coupled receptor, with the lowest activity obtained with the DMPC and POPC lipids, and the highest observed when DOPG and POPG were applied. Taken together, these results indicate that the charge of the lipid head group as well as the flexibility of the fatty acid chain are important modulators for the folding and activity of this cell membrane protein. The final nanodisc concentration can also be an important factor for membrane protein quality.For example, if the nanodisc concentration is screened within a range of 3.75 to 60 micromolar, a complete solubilization of the G protein-coupled receptor is obtained at approximately 30-micromolar nanodiscs, giving a one-to-three membrane protein-to-nanodisc ratio. In contrast, a complete solubilization of proteorhodopsin can be achieved with approximately 10-micromolar nanodiscs, giving a 10-to-one proteorhodopsin-to-nanodisc ratio. Accurate pipetting and high-quality components are certainly mandatory.Critical steps for lysate preparation are harvesting the cells in mid-log phase and applying the high-salt step. The technique drastically reduces complexity of membrane protein synthesis. Formerly challenging targets are efficiently synthesized, hence allowing to work on molecular structures of so far unexplored pharmaceutically relevant proteins.

co-translational-insertion-membrane-proteins-into-preformed

Research

JoVE Journal

Biochemistry

3.2K visualizações.  Goethe University Frankfurt. The protocol is highly efficient and versatile. It can be applied to almost any membrane protein, and the whole production procedure is completed within a day. The synthesized membrane proteins can be stabilized already co-translationally by supplied ligands.Detergent contacts are completely avoided, and the proteins insert and fold directly in provided and tailored membrane environments. Structural approaches, functional characterization, and antibody generation are prime areas of app...