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, snorkelling's functional effects are obscure. Integrin beta TMDs have such conserved basic amino acids. Here we use NMR spectroscopy(7,8) to show that integrin beta(3)(Lys 716) helps determine beta(3) TMD topography. The alpha(IIb)beta(3) TMD structure indicates that precise beta(3) TMD crossing angles enable the assembly of outer and inner membrane 'clasps' that hold the ab TMD together to limit transmembrane signalling(9). Mutation of beta(3)(Lys 716) caused dissociation of alpha(IIb)beta(3) TMDs and integrin activation. To confirm that altered topography of beta(3)(Lys 716) mutants activated alpha(IIb)beta(3), we used directed evolution of beta(3)(K716A) to identify substitutions restoring default state. Introduction of Pro(711) at the midpoint of beta(3) TMD (A711P) increased alpha(IIb)beta(3) TMD association and inactivated integrin alpha(IIb)beta(3)(A711P, K716A). beta(3)(Pro 711) introduced a TMD kink of 30 +/- 1 degrees precisely at the border of the outer and inner membrane clasps, thereby decoupling the tilt between these segments. Thus, widely occurring snorkelling residues in TMDs can help maintain TMD topography and membrane-embedding, thereby regulating transmembrane signalling.