A conserved motif in S-layer proteins is involved in peptidoglycan binding in Thermus thermophilus

Abstract

There is experimental evidence to suggest that the 100-kDa S-layer protein from Thermus thermophilus HB8 binds to the peptidoglycan cell wall. This property could be related to the presence of a region ( Paracrystalline surface layers (S-layers) are commonly found within prokaryotes belonging to different phylogenetic groups (22). As the outermost envelope, the functions of S-layers are related to their interaction with the specific environment in which these organisms grow. However, their constant presence in bacteria belonging to the oldest phylogenetic groups and the severe defects shown by S-layer-defective mutants in these groups (13, 17) suggest that primitive S-layers could have played a role essentially related to the maintenance of the cell morphology and envelope integrity (2, 22).Three-dimensional reconstruction of S-layers has revealed features common among them (22), independently of differences in their specific functions. Essentially, S-layers present a smooth surface which faces outside and a more corrugated one that binds them to the underlying envelope, whatever its nature (peptidoglycan, outer membrane, or lipopolysaccharide). At higher resolution, this corrugated side shows up as wide columns, located at the main symmetry axis, that are interconnected through a network of thin contacts at the surface. This network of contacts generates the smooth face of the S-layer.This common morphology of the S-layer is built up by a single protein component that is folded in at least two clear domains: a heavy domain, which interacts with other equivalent domains to form the wide columns that bind the S-layer to the cell, and one or more light domains to connect them at the surface (28). However, the resolution of the three-dimensional models available at present does not allow a correlation between structural topology and protein sequence to be made. In addition, the scarcity of both S-layer models and protein sequences and the high divergence found within the latter do not in general allow association of specific sequence motifs or patterns with specific structural features. However, in a recent article, Lupas et al. (20) have described the existence of one or more copies of a protein domain known as the S-layer homology (SLH) region in a number of S-layers and regular membrane proteins from unrelated bacteria. On the basis of the homology, also found with extracellular enzymes from grampositive bacteria (20), a peptidoglycan-binding function was tentatively assigned to this domain.Despite belonging to a phylogenetic group different from that of gram positive bacteria, the 100-kDa S-layer protein from Thermus thermophilus HB8 was included among those that contain an SLH domain (11,20). In this 917-amino-acidlong protein (SlpA), the SLH domain was found from amino acid positions 28 to 87, thus suggesting the possibility of an interaction in vivo between the S-layer and the peptidoglycan. Two biochemical data can be argued to support the existence of such interactions. First, the solubilization of SlpA w...

Linker mutagenesis of the Caulobacter crescentus S-layer protein: toward a definition of an N-terminal anchoring region and a C-terminal secretion signal and the potential for heterologous protein secretion

“…Recently, deletion mutagenesis (27,58), proteolytic modification of S-layer proteins (21), epitope accessibility determination and mapping (22,44), and primary sequence comparisons (41) have been applied to the study of S-layer structure. In this study, we have investigated the use of linker mutagenesis in an effort to introduce localized disruptions of S-layer proteins.…”

Molecular Control of Bacterial Death and Lysis

“…Studies with Bacillus anthracis have also demonstrated that an unspecified carbohydrate (perhaps teichoic acid) within the cell wall is subject to pyruvate modification, which is important for the recognition and binding of S-layer proteins (162). Like murein hydrolases, these proteins contain repeat elements referred to in this case as S-layer homology (SLH) domains, which are required for cell wall targeting (149,185,217). Interestingly, many of the other proteins found to contain SLH domains are murein hydrolases, suggesting that they also recognize and bind these pyruvylated carbohydrates (162).…”

S-Layer Proteins

“…By sequence comparison, S-layer-homologous (SLH) motifs (74) have been identified at the N-terminal part of many Slayer proteins (14,26,33,39,40,51,65,71,72,85,97,144) and at the C-terminal end of cell-associated exoenzymes (71,78,79) and other exoproteins (71, 73) of gram-positive bacteria. According to their origin (S-layer proteins, cell-associated exoproteins, porins), SLH motifs have been divided into three main groups whose specific properties have recently been reviewed by Engelhardt and Peters (38).…”