Transmembrane (TM) helices engage in homomeric and heteromeric interactions that play essential roles in the folding and assembly of TM proteins. However, features that explain their propensity to interact homomerically or heteromerically and determine the strength of these interactions are poorly understood. Integrins provide an ideal model system for addressing these questions because the TM helices of full-length integrins interact heteromerically when integrins are inactive, but isolated TM helices are also able to form homodimers or homooligomers in micelles and bacterial membranes. We sought to determine the features defining specificity for homointeractions versus heterointeractions by conducting a comprehensive comparison of the homomeric and heteromeric interactions of integrin alphaIIbbeta3 TM helices in biological membranes. Using the TOXCAT assay, we found that residues V700, M701, A703, I704, L705, G708, L709, L712, and L713, which are located on the same face of the beta3 helix, mediate homodimer formation. We then characterized the beta3 heterodimer by measuring the ability of beta3 helix mutations to cause ligand binding to alphaIIbbeta3. We found that mutating V696, L697, V700, M701, A703. I704, L705, G708, L712, and L713, but not the small residue-X(3)-small residue motif S699-X(3)-A703, caused constitutive alphaIIbbeta3 activation, as well as persistent focal adhesion kinase phosphorylation dependent on alphaIIbbeta3 activation. Because alphaIIb and beta3 use the same face of their respective TM helices for homomeric and heteromeric interactions, the interacting surface on each has an intrinsic "stickiness" predisposing towards helix-helix interactions in membranes. The residues responsible for heterodimer formation comprise a network of interdigitated side chains with considerable geometric complementarity; mutations along this interface invariably destabilize heterodimer formation. By contrast, residues responsible for homomeric interactions are dispersed over a wider surface. While most mutations of these residues are destabilizing, some stabilized homooligomer formation. We conclude that the alphaIIbbeta3 TM heterodimer shows the hallmark of finely tuned heterodimeric interaction, while homomeric interaction is less specific.
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