Three new members of the serine-aspartate repeat protein multigene family of Staphylococcus aureus

Three new genes encoding the serine-aspartate (SD) repeat-containing protei ns SdrC, SdrD and SdrE were found in Staphylococcus aureus strain Newman, T he SD repeats had earlier been found in the S. aureus fibrinogen-binding cl umping factors ClfA and ClfB, The clfA and clfB genes encode high-molecular -mass fibrinogen-binding proteins that are anchored to the cell surface of S. aureus, The sdr genes now reported are closely linked and tandemly array ed. The putative Sdr proteins have both organizational and sequence similar ity to ClfA and ClfB, At the N-terminus, putative secretory signal sequence s precede approximately 500 residue A regions. The A regions of the Sdr and Clf proteins exhibit only 20-30 % residue identity when aligned with any o ther member of the family. The only conserved sequence is the consensus mot if TYTFTDYVD, The Sdr proteins differ from ClfA and ClfB by having two to f ive additional 110-113 residue repeated sequences (B-motifs) located betwee n region A and the R-region, Each B-motif contains a consensus Ca2+-binding EF-hand loop normally found in eukaryotic proteins. The structural integri ty of recombinant SdrD(B1-B5) protein comprising the five B-repeats of SdrD was shown by bisANS fluorescence analysis to be Ca2+-dependent, suggesting that the EF-hands are functional. When Ca2+ was removed the structure coll apsed to an unfolded conformation. The original structure was restored by a ddition of Ca2+. The C-terminal R-domains of the Sdr proteins contain 132-1 70 SD residues. These are followed by conserved wall-anchoring regions char acteristic of many surface proteins of Cram-positive bacteria. The sdr locu s was present in all 31 S. aureus strains from human and bovine sources tes ted by Southern hybridization, although in a few strains it contained two r ather than three genes.