Lecture 5 - Bacterial Surface Proteins.pdf

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Cell and Systems Biology
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William Navarre

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MGY377H © Lisa| Page 113 L E C T U R E 5 : B A C T E R I A L S U R F A C E P R O T E I N S  the bacterial cell surface is not barren PROTEIN A OF STAPHYLOCOCCUS AUREUS  S. aureus (Gm+) has the ability to bind antibodies (immunoglobulins) produced by the immune system of mammals – helps the bacteria prevent the antibodies from working properly  protein A is the immunoglobulin-binding factor  it is displayed on the surface of the bacterial cell  protein has also been used in biotechnology – purification of antibodies away from other serum components & immunoprecipitation  S. aureus treated w antibody carrying a fluorescent label (Cy3) will fluoresce SYNTHESIS OF SURFACE PROTEINS  many surface proteins in Gm+ bacteria are synthesized w a C-terminal sorting signal  the C-terminal sorting signal contains 3 parts: 1. highly conserved LPXTG sequence (Leu, Pro, [any], Thr, Gly) 2. a stretch of mostly hydrophobic residues (~20AAs) 3. positively charged C-terminal tail of ~5 AAs (rich in Lys & Arg)  after the protein A is made (on a ribosome in the cytoplasm), it is exported from the cell  during its export, the C-terminal hydrophobic & charged domains temporarily restrain the protein in the membrane 1. sortase (a membrane-bound enzyme) recognizes the LPXTG motif of protein A & cleaves the signal bw the T & G residues  the newly formed carboxy terminus of protein A is attached to the sortase enzyme & the removed C-terminal sorting seq is degraded 2. sortase links the new C-terminus of protein A to a lipid-linked cell wall subunit (PG is made from these same lipid-linked cell wall precursors) 3. the PG precursor linked to protein A is then incorporated into the net of the cell wall by transpeptidation & transglycosylation rxs – the protein is now attached to the cell wall 4. the lipid carrier is recycled for another round of cell wall synthesis or protein anchoring MGY377H © Lisa | Page 23 TEICHOIC ACIDS (TA)  only in Gm+ bacteria ex. staphylococci, streptococci, lactobacilli, & Bacillus species  TA are polyl phosphate polymers w either ribitol or glycerol linked by phosphodiester bonds  at least 4 kinds of TAs: 1. repeat units of ribitol phosphate 2. repeat units of glycerol phosphate 3. repeat units of galactosyl glycerol phosphate 4. repeat units of N-acetyl-glucosamine phosphate  TAs are attached to the bacterial surface by a linkage unit that is attached to C6 of N-acetyl- muramic acid residues (of PG)  5-10% of the N-acetyl-muramic acid residues are attached to TA polymers  C6 carboxy group can be modified to be attached to the linkage unit  polymers are 35-40 repeating units in length but lengths may vary  negative charges due to the phosphate groups confer a –ve charge to the bacteria surface, serving as a cation-sequestering mechanism LIPOTEICHOIC ACIDS (LTA)  occurs in Gm+ bacteria only  not covalently bound to the PG (TAs are)  lipoteichoic acids are membrane teichoic acids, consisting of ribitol phosphate or glycerol phosphate repeating units terminally linked to a glycolipid (ex. gentiobiose diglyceride) which anchors the polymer in the outer face of the CM  20-30 repeat units long  protrudes through the matrix of the cell wall – exposed to the outer surface & serves as an adherence factor POLYSACCHARIDES OF STREPTOCOCCUS (Gm+)  polymers of repeating units of 2 or more neutral sugars linked to C6 of N-acetyl muramic acids  useful for serogrouping Streptococcus  are the C (carboxyhydrate) antigens of the Lancefield serogrouping system  serogroups designed A to O MGY377H © Lisa| Page 313 OUTER MEMBRANE PROTEINS (Gm-)  OM is a permeability barrier for hydrophilic solutes, including most nutrients  but it contains various proteins for the purpose of allowing the influx of nutrients & exclusion of wastes  >50% of the OM is composed of proteins  these proteins are distinct from those in the CM  there are 4-5 major types of proteins PORINS  the most abundant protein in the OM  spans the OM to form a narrow, water-filled “pore”/channel though the membrane  nutrients (generally hydrophilic) cross the OM through these pores  2 major types of porins: 1. classical porins 2. specific channels CLASSICAL PORINS  form nonspecific pores/channels for rapid passage of small hydrophilic molecules across the OM  include OmpF, OmpC, PhoE, in E. coli & the homologous proteins plus OmpD in Salmonella typhimurium  PhoE is unique in that it is produced only under conditions of phosphate starvation  classical porins form trimers  each subunit (monomer) is made by 16-β strands to form a β barrel surrounding a large channel  the diameters of the channels are 1.1-1.2 nm, allowing passage of hydrophilic molecules w MW <600-700 Da (ex. glucose, arabinose, glycerol)  under adverse conditions (poor nutrition), OmpC (1.1 nm) is replaced by OmpF which forms pores of a larger diameter (1.2 nm), allowing cells to take up larger molecules  OmpF & OmpC prefer cations slightly over anions  PhoE prefers anions  classical porins of E. coli & Salmonella: Name Regulation Present in:
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