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Staphylococcal Sortases and Surface Proteins

This chapter reviews what is known about surface proteins of Staphylococcus aureus, their mechanisms of anchoring to the cell wall envelope, and their contributions to the pathogenesis of staphylococcal infections. Protein A amino acid sequence, gene sequence, and three dimensional nuclear magnetic resonance and X-ray diffraction structures revealed a molecule comprised of five nearly identical Ig-binding domains as well as the molecular elements involved in binding Ig. S. aureus strains clump in the presence of plasma; this phenomenon, which has been exploited for diagnostic purposes, is the product of a molecular interaction between two microbial surface components recognizing adhesive matrix molecules (MSCRAMMS), clumping factor A and B, with fibrinogen. Both S. aureus and S. epidermidis strains encode for multiple cell wall-anchored surface proteins with large serine-aspartate repeat (Sdr) domains. In addition to the subset of S. aureus sortase-anchored cell wall surface proteins that are covalently attached to the cell wall, there are also a number of surface proteins that lack a C-terminal cell wall sorting signal, yet remain in one way or another cell wall associated. The current model for the uptake of heme-iron by the Isd proteins states that IsdA, IsdB, and IsdH would interact with host hemoproteins such as hemoglobin (Hb), haptoglobin (Hpt), and/or hemopexin, which are released after erythrocyte lysis. The study and characterization of these and other cell wall-associated proteins has provided much insight into the understanding of the interactions of S. aureus with its environment.