Determinants of ligand binding specificity of the alpha(1)beta(1) and alpha(2)beta(1) integrins

The alpha(1)beta(1) and alpha(2)beta(1) integrins are cell surface collagen receptors, Cells expressing the alpha(1)beta(1) integrin preferentially ad here to collagen TV, whereas cells expressing the alpha(2)beta(1) integrin preferentially adhere to collagen I. Recombinant a, and a, integrin I domai ns exhibit the same collagen type preferences as the intact integrins, Ttl addition, the alpha(2) integrin I domain binds echovirus 1; the a, I domain does not. To identify the structural components of the I domains responsib le for the varying ligand specificities, we have engineered several alpha(1 )/alpha(2) integrin I domain chimeras and evaluated their virus and collage n binding activities. Initially, large secondary structural components of t he alpha(2) I domain were replaced with corresponding regions of the a, I d omain. Following analysis in echovirus 1 and collagen binding assays, chime ras with successively smaller regions of alpha(1) I were constructed and an alyzed. The chimeras were analyzed by ELISA with several different alpha(2) integrin monoclonal antibodies to assess their proper folding. Three diffe rent regions of the alpha(1) I domain, when present in the alpha(2) I domai n, conferred enhanced collagen IV binding activity upon the alpha(2) I doma in. These include the alpha 3 and alpha 5 helices and a portion of the as h elix. Echovirus 1 binding was lost in a chimera containing the alpha C-alph a 6 loop; higher resolution mapping identified Asn(289) as playing a critic al role in echovirus 1 binding. Asn289 had not been implicated in previous echovirus 1 binding studies. Taken together, these data reveal the existenc e of multiple determinants of ligand binding specificities within the alpha (1) and alpha(2) integrin I domains.