Homo- and hetero-oligomeric interactions between the transmembrane (TM) helices of integrin α and β subunits may play an important role in integrin activation and clustering. As a first step to understanding these interactions, we used the TOXCAT assay to measure oligomerization of the wild-type αIIb TM helix and single-site TM domain mutants. TOXCAT measures the oligomerization of a chimeric protein containing a TM helix in the Escherichia coli inner membrane via the transcriptional activation of the gene for chloramphenicol acetyltransferase. We found the amount of chloramphenicol acetyltransferase induced by the wild-type αIIb TM helix was approximately half that induced by the strongly dimerizing TM helix of glycophorin A, confirming that the αIIb TM domain oligomerizes in biological membranes. Mutating each of the αIIb TM domain residues to either Ala, Leu, Ile, or Val revealed that a GXXXG motif mediates oligomerization. Further, we found that the residue preceding each glycine contributed to the oligomerization interface, as did the residue at position i + 4 after the second Gly of GXXXG. Thus, the sequence XXVGXXGGXXXLXX is critical for oligomerization of aIIb TM helix. These data were used to generate an atomic model of the αIIb homodimer, revealing a family of structures with right-handed crossing angles of 40° to 66°, consistent with a 4.0-residue periodicity, and with an interface rotated by 56° relative to glycophorin A. Thus, although the αIIb TM helix makes use of the GXXXG framework, neighboring residues have evolved to engineer its dimerization interface, enabling it to subserve specific and specialized functions.
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