THE ROLE OF THE I-DOMAIN IN LIGAND-BINDING OF THE HUMAN INTEGRIN-ALPHA(1)BETA(1)

We report here the analysis of potential ligand binding domains within the human integrin alpha(1) subunit, a known collagen/laminin recepto r. This integrin is effectively blocked by the mouse monoclonal antibo dy 1B3.1. A truncated version of the alpha 1 subunit lacking the NH2-t erminal half of the extracellular domain is not recognized by monoclon al antibody 1B3.1. Furthermore, we have isolated a cDNA containing the I domain from chicken alpha(1) bearing significant homology to the hu man and rat alpha(1) sequences. Replacing the human I domain with its chicken counterpart led to the surface expression of a functional hete rodimer with endogenous mouse beta(1) on NIH 3T3 cells. However, 1B3.1 does mot bind to the chicken/human chimera, demonstrating that the hu man alpha(1) I domain is required for epitope recognition. Mutation of Asp(253) within the I domain to alanine resulted in surface expressio n of an alpha beta heterodimer recognized by 1B3.1 but with markedly r educed binding to collagen TV or laminin. Since a previously reported mutation of a homologous Asp in the Mac-1 I domain has similar consequ ences, these results suggest a central role for the I domain in ligand recognition for all integrin alpha subunits containing this domain.