Proposal for a peptidoglycan‐associating alpha‐helical motif in the C‐terminal regions of some bacterial cell‐surface proteins

In a recent MicroCorrespondence (De Mot and Vanderleyden, 1994, Mol Microbiol 12; 333-334) an interaction of the C-terminal regions of a number of bacterial cellsurface proteins with the peptidoglycan was suggested, among them the enterobacterial OmpA proteins, pseudomonad OprF proteins, some PAL proteins, neisserial Rmp proteins, and flageliar MotB proteins. Here I wish to point out some additional facts supporting this hypothesis. First, if the C-terminal (presumably periplasmic) part of the Escherichia coli OmpA protein (amino acid residues 188-325) is analysed by a new method for secondarystructure prediction (Rost and Sander, 1993, J Moi Biol 232: 584-599), there are two a-helices strongly (Gln250-Ser-266, Glu-212-Asn-226) and one weakly (Lys294-Asp-30i) predicted. The first one represents the most conserved part of these C-terminal regions. A corresponding a-helical content comprising about 42 residues (versus 40 residues 'in this prediction) has been demonstrated for the C-terminal part of OmpA by Raman spectroscopy (Vogel and Jahnig, 1986, J Mol Bioi 190:191 -199). Both strongly predicted a-helices are identified by the same prediction algorithm in all OmpA-related proteins which are contained in the SWISSPROT database (Release 30.0). A multiple alignment together with the predicted secondary structure of all these proteins is available on request. Second, a helical wheel plot (Fig. 1) shows that all highly conserved amino acid residues would be located at one face of a putative a-helix. Hence, this consensus motif (NX2LSX2RAX2VX3L) could provide a molecular explanation for the hypothetical interaction with the peptidoglycan. Third, two E coli PAL-PhoA fusion proteins have been described (Lazzaroni and Portalier, 1992, Mol Microbioi 6i 735-742). One Is fused at Gly-116, corresponding to the C-termina! end of the predicted and conserved a-helix of PAL, and mediates a strong association with the peptidoglycan. The other is only 15 amino acid residues shorter and lacks this strong association. These results may be interpreted as indicating that a major determinant of peptidoglycan-association is located in this region. Finally, during isolation of dominant, non-functional mofS alleles of E. co//(Blair ef a/., 1991, J Bacteriol ^73: 4049-4055) a surprising clustering of mutations was observed, resulting in changes near or within the proposed cc-heiical motif. The disnjption of the interaction with the peptidoglycan provides an explanation for the observed phenotypes, as already suggested by De Mo! and Vanderleyden in their MicroCorrespondence. Database searches (GenBank Release 86.0: TFASTA) revealed additional homoiogues of this OmpA-related protein family (Gentry-Weeks ef a/., 1992, J Bacterioi 174: 7729-7742; Hardham and Stamm, 1994, Infect Immun