In Thermoanaerobacterium thermosulfurigenes EM1 S-layer homology domains do not attach to peptidoglycan

Three exocellular enzymes of Thermoanaerobacterium thermosulfurigenes EM1 p ossess a C-terminal triplicated sequence related to a domain of bacterial c ell surface proteins (S-layer proteins), At least one copy of this sequence , named the SLH (for S-layer homology) domain, is also present at the N ter minus of the S-layer protein of this bacterium. The hypothesis that SLH dom ains serve to anchor proteins to the cell surface was investigated by using the SLH domain-containing xylanase. This enzyme was isolated from T. therm osulfurigenes EM1, and different forms with and without SLH domains were sy nthesized in Escherichia coli. The interaction of these proteins with isola ted components of the cell envelope was determined to identify the attachme nt site in the cell wail. In addition, a polypeptide consisting of three SL H domains and the N terminus of the S-layer protein of T. thermosulfurigene s EM1 were included in these studies. The results indicate that SLH domains are necessary for the attachment of these proteins to peptidoglycan-contai ning sacculi. Extraction of the native sacculi with hydrofluoric acid led t o the conclusion that not peptidoglycan but accessory cell wall polymers fu nction as the adhesion component in the cell wall, Our results provide furt her evidence that attachment of proteins via their SLH domains represents a n additional mode to display polypeptides on the cell surfaces of bacteria.