Our previous work suggested that streptococcal pyrogenic exotoxin (SPE) B-induced apoptosis is mediated through a receptor-like mechanism. In this study, we have identified ␣ v  3 and Fas as the SPE B receptors for this function. The SPE B fragment without the RGD motif and G308S, a SPE B mutant with the RSD motif, induced less apoptosis than did native SPE B, suggesting that the RGD motif is critical for SPE B-induced apoptosis. Fluorescein isothiocyanate-SPE B binding assays and immunoprecipitation analysis showed that SPE B specifically interacted with ␣ v  3 . Anti-␣ v  3 antibody partially inhibited SPE B-induced apoptosis but had no effect on G308S-induced apoptosis. In addition, Fas binding to SPE B was verified in an affinity column and an immunoprecipitation analysis. Anti-Fas antibody inhibited SPE B-and G308S-induced apoptosis in a dose-dependent manner, suggesting that Fas-mediated SPE B-induced apoptosis also occurs RGD independently. Both anti-␣ v  3 and anti-Fas antibodies synergistically inhibited SPE B-induced apoptosis. The apoptotic cascades were activated by SPE B and G308S, with a little delay by the latter. After SPE B binding, the cell surface level of ␣ v  3 , but not of Fas, was decreased. The decreased ␣ v  3 level was restored by treatment with the proteasome inhibitor MG132, suggesting a SPE B-mediated endocytosis of integrin ␣ v  3 via the ubiquitin-proteasome system. Taken together, our results demonstrate that SPE B-induced apoptosis is mediated through ␣ v  3 integrin and Fas in a synergistic manner.Streptococcal pyrogenic exotoxin (SPE) B is an important virulence factor produced by group A streptococcus (GAS), a human pathogenic bacterium that causes infections of various severities (30). SPE B is a cysteine protease secreted as a 40-kDa zymogen that is autocatalytically converted to a 28-kDa mature form. Studies of cellular reactions of SPE B have focused on its protease activity (10,15,43). Its role as a ligand protein has not been critically evaluated. Our recent study showed that SPE B-induced apoptosis in human lung epithelial A549 cells is mediated through a receptor-like mechanism and a mitochondrion-dependent pathway and that the protease activity of SPE B is required to initiate apoptotic signaling, most likely by exposing the binding site for SPE B (44). However, the SPE B receptors and the molecular mechanism of SPE B interaction with its receptors remain to be explored.Three main SPE B variants have been identified in 200 GAS isolates of the speB gene (39). One variant contains an arginineglycine-aspartic acid (RGD) sequence and interacts with human integrins ␣ v  3 and ␣ IIb  3 . The GAS isolates that contain the RGD motif in SPE B are the most common cause of invasive diseases. Moreover, Kagawa et al. (23) demonstrated that the surface location of an RGD motif is a feature unique to SPE B among cysteine proteases and is linked to the pathogenesis of the most invasive strains of GAS. Therefore, understanding the role of the RGD motif in the interaction of...