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Envelope proteins

Other envelope proteins

A whole variety of other chlamydial envelope proteins remain to be characterised. Those whose function can be reasonably inferred from database searches include pbpb and its paralogue penA, a penicillin binding protein which has been associated with a reduced affinity / susceptibility to penicillin in Neisseria, plus a  whole set of genes for peptidoglycan and lipopolysaccharide biosynthesis. A list of potential outer membrane proteins is shown in Table 1 [based on Stephens & Lammel, 2000]. Those interested in pursuing these genes more fully should visit the useful Los Alamos STD Database at:  http://www.stdgen.lanl.gov/ select the chlamydial species of interest, then place a <functional class> and then <cell envelope> search. Experimental proof of function is, however, still in many cases required.

Table 1: Predicted outer membrane proteins of C. trachomatis and C. pneumoniae. Modified from: Stephens & Lammel, 2000.

One group has adopted a combined genomic-proteomic approach to identifying chlamydial surface proteins, making use of: (i) in silico prediction of peripherally located proteins from available genome sequences; (ii) the expression and purification of selected proteins in non-chlamydial systems then; (iii) the production of mouse immune sera against the recombinant proteins for Western blot and fluorescence-activated cell sorter (FACS) analyses for the identification of surface antigens, and finally; (iv) mass spectrometric identification of FACS-positive surface antigens. Using this approach, of 53 FACS-positive sera, 41 recognized a protein species of the expected size on Western blot, while 28 of the 53 antigens were identified by mass spectrometry on two dimensional gel electrophoresis maps of chlamydial elementary-body extracts. It was claimed that this is the first systematic attempt to define surface protein organization in C. pneumoniae [Montigiani et al., 2002]. See also the Aarhus University proteome database [van Dahl et al., 2001 & Shaw et al., 2002] and the work of the Siena proteomics group [Giannikopoulou et al., 1997; Sanchez-Campillo et al., 1999].

[MEW] March 2003

[ MEW comment: At the Memphis CBRS meeting March 2003, Christine Wu (Scripps Institute) presented a powerful liquid phase shotgun method of proteomic analysis using formic acid CNBr digestion, mass spectroscopy and bioinformatics. An impressive 384 unique EB components and 27 outer membrane (COMC) components have been identified together with other 100 phosphorylation sites that might be important for signalling and regulation. On MOMP, both phosphotyrosine and phosphoserine had been identified, with the phosphotyrosine apparently located on the MOMP adjacent to the periplasmic space while the phosphoserine was on the external surface of the protein in a suitable location for interaction with host cells. Phosphorylation sites such as these are likely to be important for signalling and regulation. For an example of this high-throughput technique see Florens et al., 2002].

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