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...

The S-layers of Thermus thermophilus HB27 and T. thermophilus HB8 are composed of protein units of 95 kDa (P95) and 100 kDa (P100), respectively. We have selected S-layer deletion mutants from both strains by complete replacement of the slpA gene. Mutants of the two strains showed similar defects in growth and morphology and overproduced an external cell envelope inside of which cells remained after division. However, the nature of this external layer is strain specific, being easily stained and regular in the HB8⌬slpA derivative and amorphous and poorly stained in the HB27⌬slpA strain. The addition of chromosomic DNA from T. thermophilus HB8 to growing cultures of T. thermophilus HB27⌬slpA led to the selection of a new strain, HB27C8, which expressed a functional S-layer composed of the P100 protein. Conversely, the addition of chromosomic DNA from T. thermophilus HB27 to growing cultures of T. thermophilus HB8⌬slpA allowed the isolation of strain HB8C27, which expressed a functional S-layer composed of the P95 protein. The driving force which selected the transference of the S-layer genes in these experiments was the difference in growth rates, one of the main factors leading to selection in natural environments.In many mesophilic bacteria, the presence of crystalline surface layers (S-layers) seems to be a strain-specific character, whose loss upon optimal growth conditions results in S-layer mutants that do not present any apparent phenotypic defect (12, 21). By contrast, most thermophiles, especially those belonging to the oldest phylogenetic branches (31), contain Slayer as an almost universal character, thus suggesting for these structures important roles in cell viability or membrane integrity at high temperatures. In fact, the presence of S-layer in evolutionarily old thermophilic bacteria and their structural simplicity led to the suggestion of an ancient evolutionary origin for such structures (26).As S-layers completely surround the cells, their building units constitute one of the major membrane proteins. Accordingly, synthesis of the S-layer is a metabolically expensive process which could be selected during evolution only as a result of the existence of strong selective pressures. However, little is known about such selective pressures, and even less is known about the role(s) that S-layers could play in mesophilic or thermophilic natural environments.Like any cell surface components, S-layers should be subjected to strong selective pressures, the most important of which could be the presence of hydrolytic enzymes (essentially proteases) and bacteriophages (as binding sites). Such selective pressures could be the factors responsible for the sequence divergence of the S-layer genes (21). In fact, it has been not possible to obtain clear phylogenetic relationships between S-layer genes, even from related organisms. In addition, the possibility exists that horizontal transference of S-layer genes within genetically related strains contributed to the present sequence divergence, in a way similar to th...

In this chapter we report on the molecular biology of crystalline surface layers of different bacterial groups. The limited information indicates that there are many variations on a common theme. Sequence variety, antigenic diversity, gene expression, rearrangements, influence of environmental factors and applied aspects are addressed. There is considerable variety in the S-layer composition, which was elucidated by sequence analysis of the corresponding genes. In Corynebacterium glutamicum one major cell wall protein is responsible for the formation of a highly ordered, hexagonal array. In contrast, two abundant surface proteins from the S-layer of Bacillus anthracis. Each protein possesses three S-layer homology motifs and one protein could be a virulence factor. The antigenic diversity and ABC transporters are important features, which have been studied in methanogenic archaea. The expression of the S-layer components is controlled by three genes in the case of Thermus thermophilus. One has repressor activity on the S-layer gene promoter, the second codes for the S-layer protein. The rearrangement by reciprocal recombination was investigated in Campylobacter fetus. 7-8 S-layer proteins with a high degree of homology at the 5' and 3' ends were found. Environmental changes influence the surface properties of Bacillus stearothermophilus. Depending on oxygen supply, this species produces different S-layer proteins. Finally, the molecular bases for some applications are discussed. Recombinant S-layer fusion proteins have been designed for biotechnology.