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...