Structure of Collagen Receptor Integrin alpha I-1 Domain Carrying the Activating Mutation E317A

Abstract

We have analyzed the structure and function of the integrin alpha I-1 domain harboring a gain-of-function mutation E317A. To promote protein crystallization, a double variant with an additional C139S mutation was used. In cell adhesion assays, the E317A mutation promoted binding to collagen. Similarly, the double mutation C139S/E317A increased adhesion compared with C139S alone. Furthermore, soluble alpha I-1 C139S/E317A was a higher avidity collagen binder than alpha I-1 C139S, indicating that the double variant represents an activated form. The crystal structure of the activated variant of alpha I-1 was solved at 1.9 angstrom resolution. The E317A mutation results in the unwinding of the alpha C helix, but the metal ion has moved toward loop 1, instead of loop 2 in the open alpha I-2. Furthermore, unlike in the closed alpha I domains, the metal ion is pentacoordinated and, thus, prepared for ligand binding. Helix 7, which has moved downward in the open alpha I-2 structure, has not changed its position in the activated alpha I-1 variant. During the integrin activation, Glu(335) on helix 7 binds to the metal ion at the metal ion-dependent adhesion site (MIDAS) of the beta(1) subunit. Interestingly, in our cell adhesion assays E317A could activate collagen binding even after mutating Glu(335). This indicates that the stabilization of helix 7 into its downward position is not required if the alpha(1) MIDAS is already open. To conclude, the activated alpha I-1 domain represents a novel conformation of the alpha I domain, mimicking the structural state where the Arg(287)-Glu(317) ion pair has just broken during the integrin activation.