An oxygen-induced but protein F-independent fibronectin-binding pathway in Streptococcus pyogenes

Protein F is an important fibronectin-binding adhesin of Streptococcus pyogenes (group A streptococcus). However, all previous analyses of protein F have been conducted in a mutant strain which expresses protein F under anaerobic conditions nonpermissive for expression in other strains. In this study, we have examined the fibronectin-binding properties of several protein F-deficient mutants cultured under aerobic conditions and have identified a second pathway for binding fibronectin. Unlike the case with protein F, exposure to an aerobic environment does not induce transcription of a new gene product. Rather, O₂ is apparently required for the modification of a protease-resistant cell surface component into a binding-competent form. Modification occurred preferentially at a pH of 6.0 or less, and the binding of the modified component to fibronectin required Zn²⁺. The oxidizing agent Fe(CN)₆ could be substituted for O₂ and stimulated expression of binding activity under O₂-limiting conditions. Streptococcal fibronectin binding mediated by this pathway but not by protein F could be inhibited by laminin and by streptococcal lipoteichoic acid, a molecule previously implicated as the streptococcal adhesin for fibronectin. The non-protein F-binding activity could also substantially enhance the binding of the organism to basement membrane. By using differential inhibition, analyses of binding to non- protein F mutant strains demonstrated that the total level of fibronectin bound under aerobic conditions reflects contributions from both pathways. Because of its dependence on Zn²⁺, an oxidant, and pH, this binding activity has been designated the ZOP binding pathway.