Transferrin binding in Staphylococcus aureus: involvement of a cell wall-anchored protein

The ability to gain access to iron is pivotal for bacterial pathogens during infection. Although much is known about iron acquisition systems in Gram-negative bacteria, comparatively little is known about how Gram-positive pathogens access iron from host iron sources. A previous study showed that, in the Gram-positive human pathogen Staphylococcus aureus, a cell surface-associated glyceraldehyde-3-phosphate dehydrogenase (GAPDH) enzyme (Gap, or Tpn) is capable of binding human transferrin, representing a potential means by which this bacterium is able to access iron in vivo. We have investigated this property of S. aureus further and shown that, in S. aureus RN6390, GAPDH is expressed on the S. aureus cell surface independent of exogenous iron concentrations, and that overexpressed and purified Gap, although retaining GAPDH activity, has no affinity for human transferrin. Moreover, although a S. aureus gap mutant was devoid of surface-associated and cytoplasmic GAPDH activity, it retained the ability to bind human transferrin, equivalent to wild type. We concluded from these results that the Gap protein is not involved in S. aureus binding to human transferrin. We identified the transferrin-binding protein as a novel cell wall-anchored protein, designated StbA for staphylococcal transferrin-binding protein A, which shared no significant similarities with any other bacterial transferrin-binding proteins. StbA contained a C-terminal cell wall-anchoring motif (LPKTG), and expression of StbA in the cell wall was strictly controlled by exogenous iron concentrations. The stbA gene is found within a 7 kb region in the S. aureus chromosome that contains a total of six iron-regulated genes. Immediately downstream from stbA is an iron-regulated gene whose product was predicted to be another cell wall-anchored protein with no significant similarity to proteins with characterized functions. Transcribed in the opposite direction from stbA is a four-gene operon whose expression is also regulated by iron. While the deduced products of the first two genes lack similarity to known proteins, the last two genes encode, respectively, putative lipoprotein and permease components of an ABC transporter that shares significant similarities with several iron(III) ABC transporters in a variety of bacteria.