Serum opacity factor (SOF) is a large surface protein of Streptococcus pyogenes that opacifies mammalian serum. Previous work indicated that inactivation of sof reduced the virulence of S. pyogenes. However, SOF and Sfbx were recently found to be co-expressed from the same message and, therefore, insertional inactivation of sof should also inactivate sfbx. Thus, the relative roles of SOF and Sfbx in streptococcal virulence are not clear. Herein, we report that SOF, but not Sfbx, is required for virulence in a mouse model of infections. Neither SOF nor Sfbx appear to contribute significantly to virulence in a chicken embryo model of infection. Antibodies to Sfbx were found in human sera in conjunction with antibodies to SOF suggesting that these proteins are co-expressed during infections. The sfbx genes from M types 2, 4, and 28 of S. pyogenes were cloned, and their sequences compared to those from other serotypes. An RGD sequence, an integrin-binding motif, was found in all Sfbx. However, microtiter wells coated with Sfbx did not support adhesion of HeLa tissue culture cells, suggesting that the RGD motif of Sfbx does not mediate adhesion of epithelial cells.
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