A streptococcal collagen-like protein interacts with the α₂β₁ integrin and induces intracellular signaling

The streptococcal collagen-like proteins Scl1 and Scl2 are prokaryotic members of a large protein family with domains containing the repeating amino acid sequence (Gly-Xaa-Yaa)_n that form a collagen-like triple-helical structure. Here, we test the hypothesis that Scl variant might interact with mammalian collagen-binding integrins. We show that the recombinant Scl protein p176 promotes adhesion and spreading of human lung fibroblast cells through an α₂β₁ integrin-mediated interaction as shown in cell adhesion inhibition assays using anti-α₂β₁ and anti-β₁ integrins monoclonal antibodies. Accordingly, C2C12 cells stably expressing α₂β₁ integrin as the only collagen-binding integrin show productive cell adhesion activities on p176 that can be blocked by an anti-α₂β₁ integrin antibody. In addition, p176 promotes tyrosine phosphorylation of p125 ^FAK of C2C12 cells expressing α₂β₁ integrin, whereas parental C2C12 cells do not. Furthermore, adhesion of human lung fibroblast cells to p176 induces phosphorylation of p125^FAK, p130^CAS, and p68^Paxillin proteins. In a domain swapping experiment, we show that integrin binds to the collagenous domain of the Scl protein. Moreover, the recombinant inserted domain of the α₂ integrin interacts with p176 with a relatively high affinity (K_D = 17 nM). Attempts to identify the integrin sites in p176 suggest that more than one site may be involved. These studies, for the first time, suggest that the collagen-like proteins of prokaryotes retained not only structural but also functional characteristics of their eukaryotic counterparts.