Structural Basis of Collagen Recognition by Integrin α2β1

and ␣X) contain an additional domain (" A " or " I ") that is inserted into the head region, where it plays a central role in ligand binding: thus, recombinant I domains reca-pitulate many of the ligand-binding properties of the The first crystal struc-United Kingdom † Department of Biochemistry ture of an I domain showed that it adopts the " dinucleo-tide-binding " fold, with a central mostly parallel ␤ sheet University of Cambridge Tennis Court Road surrounded on both sides by amphipathic ␣ helices (Lee et al., 1995a). At the C-terminal end of the ␤ sheet is a Cambridge CB2 1QW United Kingdom conserved metal binding site that we have called the metal ion–dependent adhesion site or MIDAS motif. Mu-‡ The Burnham Institute 10901 North Torrey Pines Road tagenesis studies show that the MIDAS motif and exposed side chains on the surrounding surface are re-La Jolla, California 92037 quired for ligand binding, and are thus likely to form the ligand contact sites (Michishita et al. We have determined the crystal structure of a complex between the I domain of integrin ␣2␤1 and a triple two different crystal forms of the ␣M-I domain led us to propose that affinity regulation occurred via changes in helical collagen peptide containing a critical GFOGER motif. Three loops on the upper surface of the I domain metal coordination at the MIDAS motif that were linked to tertiary changes in the domain (Lee et al., 1995b). that coordinate a metal ion also engage the collagen, with a collagen glutamate completing the coordination The ␣2␤1 integrin is expressed on several different cell types, where it is a receptor for collagen and laminin. sphere of the metal. Comparison with the unliganded I domain reveals a change in metal coordination linked It is required for the arrest of platelets under conditions of blood flow on the collagen fiber surface exposed as a to a reorganization of the upper surface that together create a complementary surface for binding collagen. consequence of injury (Sixma et al., 1997). Recombinant ␣2-I domain exhibits specific binding to the fibrillar colla-Conformational changes propagate from the upper surface to the opposite pole of the domain, suggesting gens and, like the complete receptor, binds in a cation-dependent manner, being supported by magnesium or both a basis for affinity regulation and a pathway for signal transduction. The structural features observed manganese but not by calcium (Tuckwell et …