Abstract

Surface (S)-layers, para-crystalline arrays of protein, are deposited in the envelope of most bacterial species. These surface organelles are retained in the bacterial envelope through the non-covalent association of proteins with cell wall carbohydrates. Bacillus anthracis, a Gram-positive pathogen, produces S-layers of the protein Sap, which uses three consecutive repeats of the surface-layer homology (SLH) domain to engage secondary cell wall polysaccharides (SCWP). Using x-ray crystallography, we reveal here the structure of these SLH domains, which assume the shape of a three-prong spindle. Each SLH domain contributes to a three-helical bundle at the spindle base, whereas another α-helix and its connecting loops generate the three prongs. The inter-prong grooves contain conserved cationic and anionic residues, which are necessary for SLH domains to bind the B. anthracis SCWP. Modeling experiments suggest that the SLH domains of other S-layer proteins also fold into three-prong spindles and capture bacterial envelope carbohydrates by a similar mechanism.