Evidence That the Integrin β3 and β5 Subunits Contain a Metal Ion-dependent Adhesion Site-like Motif but Lack an I Domain*

Abstract

The amino-terminal domain of each integrin β subunit is hypothesized to contain an ion binding site that is key to cell adhesion. A new hypothesis regarding the structure of this site is suggested by the crystallization of the I domains of the integrin α_L and α_M subunits (Lee, J.-O., Rieu, P., Arnaout, M. A., and Liddington, R. (1995) Cell 80, 631–638; Qu, A., and Leahy, D. J. (1995) Proc. Natl. Acad. Sci. U. S. A. 92, 10277–10281). In those proteins, an essential metal ion is bound by a metal ion-dependent adhesion site (MIDAS). The MIDAS is presented at the apex of a larger protein module called an I domain. The metal ligands in the MIDAS can be separated into three distantly spaced clusters of oxygenated residues. These three coordination sites also appear to exist in the integrin β3 and β5 subunits. Here, we examined the putative metal binding site within β3 and β5 using site-directed mutagenesis and ligand binding studies. We also investigated the fold of the domain containing the putative metal binding site using the PHD structural algorithm. The results of the study point to the similarity between the integrin β subunits and the MIDAS motif at two of three key coordination points. Importantly though, the study failed to identify a residue in either β subunit that corresponds to the second metal coordination group in the MIDAS. Moreover, structural algorithms indicate that the fold of the β subunits is considerably different than the I domains. Thus, the integrin β subunits appear to present a MIDAS-like motif in the context of a protein module that is structurally distinct from known I domains.