Abstract
The Sbi protein of Staphylococcus aureus comprises two IgG-binding domains similar to those of protein A and a region that triggers the activation of complement C3. Sbi is expressed on the cell surface but its C-terminal domain lacks motifs associated with wall or membrane anchoring of proteins in Gram-positive bacteria. Cell-associated Sbi fractionates with the cytoplasmic membrane and is not solubilized during protoplast formation. S. aureus expressing Sbi truncates of the C-terminal Y domain allowed identification of residues that are required for association of Sbi with the membrane. Recombinant Sbi bound to purified cytoplasmic membrane material in vitro and to purified lipoteichoic acid. This explains how Sbi partitions with the membrane in fractionation experiments yet is partially exposed on the cell surface. An LTA-defective mutant of S. aureus had reduced levels of Sbi in the cytoplasmic membrane.
Highlights
Staphylococcus aureus permanently colonizes the moist squamous epithelium of the anterior nares of approximately 20% of the population while the remainder carry the organism intermittently (Williams, 1963, Peacock et al, 2001)
The ability of S. aureus to cause infections is in part due to proteins that are anchored to the cell surface and to those that are secreted into the medium
In order to determine if the C3 binding domains of second binding protein for immunoglobulins (Sbi) were exposed on the cell surface whole cells were immobilized on membranes and probed with rabbit antibodies raised against the non-IgG binding D3D4 domains of Sbi
Summary
Staphylococcus aureus permanently colonizes the moist squamous epithelium of the anterior nares of approximately 20% of the population while the remainder carry the organism intermittently (Williams, 1963, Peacock et al, 2001). The ability of S. aureus to cause infections is in part due to proteins that are anchored to the cell surface and to those that are secreted into the medium. Among the latter are cytolytic toxins, enzymes and proteins with immune evasion functions that interfere with neutrophil migration and complement fixation (Foster, 2005). While a major function of surface-anchored proteins is to act as adhesins and invasins (Foster, 2005), it is clear that several can help the bacterium evade innate immune responses. Protein A binds to the Fc region of IgG and coats the cell with antibody that cannot be recognized by Fc receptors on neutrophils and cannot catalyze complement fixation. Clumping factor A binds fibrinogen and fibrin (McDevitt et al, 1997) but it can capture and activate the complement regulatory protease factor I which results in enhanced degradation of C3b (Cunnion et al, 2004, Hair et al, 2008)
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