Transmembrane helices that form two opposite homodimeric interactions: An asparagine scan study of αM and β2 integrins

Integrins are α/β heterodimers, but recent in vitro and in vivo experiments also suggest an ability to associate through their transmembrane domains to form homomeric interactions. While the results of some in vitro experiments are consistent with an interaction mediated by a GxxxG-like motif, homooligomers observed after in vivo cross-linking are consistent with an almost opposite helix-helix interface. We have shown recently that both models of interaction are compatible with evolutionary conservation data, and we predicted that the a-helices in both models would have a similar rotational orientation. Herein, we have tested our prediction using in vitro asparagine scan of five consecutive residues along the GxxxG-like motif of the transmembrane domain of α and β integrins, αM and β2. We show that Asn-mediated dimerization occurs twice for every turn of the helix, consistent with two almost opposite forms of interaction as suggested previously for αIIb and β3 transmembrane domains. The orientational parameters helix tilt and rotational orientation of each of these two Asn-stabilized dimers were measured by site-specific infrared dichroism (SSID) in model lipid bilayers and were found to be consistent with our predicted computational models. Our results highlight an intrinsic tendency for integrin transmembrane α-helices to form two opposite types of homomeric interaction in addition to their heteromeric interactions and suggest that integrins may form complex and specific networks at the transmembrane domain during function.