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

Abstract

Summary 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.

Characterization of novel LPXTG-containing proteins of Staphylococcus aureus identified from genome sequences

“…The sasE and sasJ genes encode proteins that have been implicated in transferrin binding (Taylor & Heinrichs, 2002). They lie adjacent to and divergent from an iron-regulated operon containing the gene for sortase B (Mazmanian et al, 2002).…”

“…The ability to bind to host extracellular matrix and plasma components promotes adhesion to host tissues, evasion of host defence mechanisms and invasion of epithelial and endothelial cells (Foster & Höök, 1998;Nilsson et al, 1998;Peacock et al, 1999). More recently, surface proteins have also been shown to play roles in biofilm formation (Cucarella et al, 2001) and binding of host transferrin (Taylor & Heinrichs, 2002).…”

Mycobacterium tuberculosis acquires iron by cell-surface sequestration and internalization of human holo-transferrin

“…Several pathogens including Staphylococcus aureus, S. epidermidis, N. meningitidis, N. gonorrhoeae, H. influenza and others are known to acquire iron by the direct binding of transferrin on the cell surface [21][22][23][24] . However, no in vivo or in vitro studies have ever identified the existence of transferrin-binding proteins on the surface of M.tb.…”

Host Airway Proteins Interact withStaphylococcus aureusduring Early Pneumonia

“…A number of S. aureus microbial surface components recognizing adhesive matrix molecules have been shown in vitro to interact with proteins of the host extracellular matrix, including fibronectin, collagen, elastin, vitronectin, and laminin (for a review, see reference 4). IsdA is a multifunctional iron-regulated microbial surface component recognizing adhesive matrix molecules that has been shown to bind to fibronectin, fibrinogen, fetuin, hemoglobin, and transferrin (6,31,43). IsdB and IsdH/HarA have also been shown to bind hemoglobin and haptoglobin (6,9,31,44).…”