LIGAND RECOGNITION BY THE I-DOMAIN-CONTAINING INTEGRINS

Seven of the integrin a subunits described to date, alpha(1), alpha(2) , alpha(L), alpha(X), alpha(d), alpha(M) and alpha(E), contain a highl y conserved I (or A) domain of approximately 200 amino acid residues i nserted near the amino-terminus of the subunit. As the result of a var iety of independent experimental approaches, a large body of data has recently accumulated that indicates that the I domains are independent , autonomously folding domains capable of directly binding ligands tha t play a necessary and important role in ligand binding by the intact integrins. Recent crystallographic studies have elucidated the structu res of recombinant alpha(M) and alpha(L) I domains and also delineated a novel divalent cation-binding motif within the I domains (metal ion -dependent adhesion site, MIDAS) that appears to mediate the divalent cation binding of the I domains and the I domain-containing integrins to their ligands.