Snorkeling Basic Amino Acid Side Chains Regulate Transmembrane Integrin Signalling

Side chains of Lys/Arg near transmembrane domain (TMD)(1–3) membrane-water interfaces can “snorkel” placing their positive charge near negatively-charged phospholipid head groups(4–6); however, snorkeling's functional effects are obscure. Integrin β TMDs exhibit such conserved basic amino acids; here we used nuclear magnetic resonance (NMR) spectroscopy(7, 8) to show that integrin β3(Lys716) helps determine β3 TMD topography. The αIIbβ3 TMD structure suggests that precise β3 TMD crossing angles enable the assembly of outer and inner membrane “clasps” (OMC and IMC) that hold the αβ TMD together to limit transmembrane signalling(9) . Mutation of β3(Lys716) caused dissociation of αIIbβ3 TMDs and integrin activation. To confirm that altered topography of β3(Lys716) mutants activated αIIbβ3, we utilized directed evolution of β3(K716A) to identify substitutions restoring default state. Introduction Pro(711) at the midpoint of β3 TMD (A711P) increased αIIbβ3 TMD association and inactivated integrin αIIbβ3(A711P,K716A). β3(Pro711) introduced a TMD kink of 30 ± 1° precisely at the OMC/IMC border, thereby decoupling the tilt between these segments. Thus, widely-occurring snorkeling residues in TMDs can help maintain TMD topography and membrane-embedding thereby regulating transmembrane signalling.