Published: May 23rd, 2021
Here we present a protocol for the rapid identification of proteins produced by genomically sequenced pathogenic bacteria using MALDI-TOF-TOF tandem mass spectrometry and top-down proteomic analysis with software developed in-house. Metastable protein ions fragment because of the aspartic acid effect and this specificity is exploited for protein identification.
This protocol identifies the immunity proteins of the bactericidal enzymes: colicin E3 and bacteriocin, produced by a pathogenic Escherichia coli strain using antibiotic induction, and identified by MALDI-TOF-TOF tandem mass spectrometry and top-down proteomic analysis with software developed in-house. The immunity protein of colicin E3 (Im3) and the immunity protein of bacteriocin (Im-Bac) were identified from prominent b- and/or y-type fragment ions generated by the polypeptide backbone cleavage (PBC) on the C-terminal side of aspartic acid, glutamic acid, and asparagine residues by the aspartic acid effect fragmentation mechanism. The software rapidly scans in silico protein sequences derived from the whole genome sequencing of the bacterial strain. The software also iteratively removes amino acid residues of a protein sequence in the event that the mature protein sequence is truncated. A single protein sequence possessed mass and fragment ions consistent with those detected for each immunity protein. The candidate sequence was then manually inspected to confirm that all detected fragment ions could be assigned. The N-terminal methionine of Im3 was post-translationally removed, whereas Im-Bac had the complete sequence. In addition, we found that only two or three non-complementary fragment ions formed by PBC are necessary to identify the correct protein sequence. Finally, a promoter (SOS box) was identified upstream of the antibacterial and immunity genes in a plasmid genome of the bacterial strain.
Analysis and identification of undigested proteins by mass spectrometry is referred to as the top-down proteomic analysis1,2,3,4. It is now an established technique that utilizes electrospray ionization (ESI)5 and high-resolution mass analyzers6, and sophisticated dissociation techniques, e.g., electron transfer dissociation (ETD), electron capture dissociation (ECD)7, ultraviolet photo-dissociation (UV-PD)8, etc.
1. Microbiological sample preparation
Figure 3 (top panel) shows the MS of STEC O113:H21 strain RM7788 cultured overnight on LBA supplemented with 400 ng/mL mitomycin-C. Peaks at m/z 7276, 7337, and 7841 had been identified previously as cold-shock protein C (CspC), cold-shock protein E (CspE), and a plasmid-borne protein of unknown function, respectively33. The protein ion at m/z 9780 [M+H]+ was analyzed by MS/MS-PSD as shown in Figure 3 (bottom panel). The p.......
The primary strengths of the current protocol are its speed, simplicity of sample preparation, and use of an instrument that is relatively easy to operate, be trained on, and maintain. Although bottom-up and top-down proteomic analysis by liquid chromatography-ESI-HR-MS are ubiquitous and far superior in many respects to top-down by MALDI-TOF-TOF, they require more time, labor, and expertise. Instrument complexity can often affect whether certain instrument platforms are lik.......
Protein Biomarker Seeker software is freely available (at no cost) by contacting Clifton K. Fagerquist at firstname.lastname@example.org. We wish to acknowledge support of this research by ARS, USDA, CRIS grant: 2030-42000-051-00-D.....
|4000 Series Explorer software
|4800 Plus MALDI TOF/TOF Analyzer
|Acetonitrile Optima LC/MS grade
|BSL-2 biohazard cabinet
|The Baker Company
|Data Explorer software
|Focus Protein Reduction-Alkylation kit
|Microcentrifuge Tubes, 2 mL, screw-cap, O-ring
|MiniSpin Plus Centrifuge
|Mitomycin-C (from streptomyces)
|Shaker MaxQ 420HP Model 420
|Sterile 1 uL loops
|Thioredoxin (E. coli, recombinant)
|Water Optima LC/MS grade
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