Partially overlapping substrate specificities of staphylococcal group A sortases.

Sortases catalyze the covalent attachment of proteins with a C-terminal LPxTG motif to the cell walls of Gram-positive bacteria. Here, we show that deletion of the srtA genes of Staphylococcus aureus and Staphylococcus epidermidis resulted in the dislocation of several LPxTG proteins from the cell wall to the growth medium. Nevertheless, proteomics and Western blotting analyses revealed that substantial amounts of the identified proteins remained cell wall bound through noncovalent interactions. The protein dislocation phenotypes of srtA mutants of S. aureus and S. epidermidis were reverted by ectopic expression of srtA genes of either species. Interestingly, S. epidermidis contains a second sortase A, which was previously annotated as ``SrtC.'' Ectopic expression of this SrtC in srtA mutant cells reverted the dislocation of some, but not all, cell wall associated proteins. Similarly, defects in biofilm formation were reverted by ectopic expression of SrtC in some, but not all, tested srtA mutant strains. Finally, overexpression of SrtA resulted in increased levels of biofilm formation in some tested strains. Taken together, these findings show that the substrate specificities of SrtA and SrtC overlap partially, and that sortase levels may be limiting for biofilm formation in some staphylococci.