The serine-aspartate repeat (Sdr) protein family in Staphylococcus epidermidis

Staphylococcus epidermidis can express three different cell-surface-associa ted proteins, designated SdrF, SdrG and SdrH, that contain serine-aspartate dipeptide repeats. Proteins SdrF and SdrG are similar in sequence and stru ctural organization to the Sdr proteins of Staphylococcus aureus and compri se unique 625- and 548-residue A regions at their N termini, respectively, followed by 110-119-residue B-repeat regions and SO-repeat regions. The C t ermini contain LPXTG motifs and hydrophobic amino acid segments characteris tic of surface proteins covalently anchored to peptidoglycan. In contrast, SdrH has a short 60-residue A region at its N terminus followed by a SO-rep eat region, a unique 277-residue C region and a C-terminal hydrophobic segm ent. SdrH lacks a LPXTG motif. Recombinant proteins representing the A regi ons of SdrF, SdrG and SdrH were expressed and purified from Escherichia col i. Antisera specific to these proteins were raised in rabbits and used to i dentify Sdr proteins expressed by S. epidermidis. Only SdrF was released fr om lysostaphin-generated protoplasts of cells grown to late-exponential pha se. SdrG and SdrH remained associated with the protoplast fraction and thus appear to be ineffectively sorted along the conventional pathway used for cell-wall-anchored proteins. In Southern hybridization analyses, the sdrG a nd sdrH genes were present in all 16 strains tested, whilst sdrF was presen t in 12 strains. Antisera from 16 patients who had recovered from S. epider midis infections contained antibodies that reacted with recombinant A regio ns of SdrG and SdrH, suggesting that these proteins can be expressed during infection.