Two functional active conformations of the integrin α2β1, depending on activation condition and cell type

Abstract

For several integrins, the existence of multiple conformational states has been studied intensively. For the integrin α2β1, a major collagen receptor on platelets and other cell types, however, no such experimental data were available thus far. Recently, our group has developed a monoclonal antibody IAC-1 sensitive to the molecular conformation of α2β1 because it only binds to the activated state of α2β1 on platelets, induced upon inside-out signaling. By investigating IAC-1 binding in combination with collagen binding after inside-out stimulation and outside manipulation, we demonstrated the existence of three different conformations of α2β1 on platelets and Chinese hamster ovary cells as follows: (i) a nonactivated, resting state with no collagen nor IAC-1 binding; (ii) an intermediate state, induced by outside manipulation, with collagen but no IAC-1 binding; and (iii) a fully activated state, induced after inside-out stimulation, with both collagen and IAC-1 binding. Moreover, these different conformational states of α2β1 are dependent on the cell type where α2β1 is expressed, as IAC-1 binding to peripheral blood mononuclear cells and Jurkat cells could also be induced by outside manipulation, in contrast to platelets and α2β1-expressing Chinese hamster ovary cells. Finally, we revealed a functional relevance for these different conformational states because the conformation of α2β1, induced after outside manipulation, resulted in significantly more cell spreading on coated collagen compared with nonactivated or inside-out stimulated cells. © 2005 by The American Society for Biochemistry and Molecular Biology, Inc.