TM2-TM7 interaction in coupling movement of transmembrane helices to activation of the angiotensin II type-1 receptor.

Agonist-induced rigid body motion of transmembrane (TM) helices has been established as a unifying mechanism in the activation of the G protein-coupled receptors. In attempts to measure specific conformational transitions during the activation of the type 1 receptor for angiotensin II (AT(1)), we found a decrease in accessibility of Cys(76) in the second TM helix, suggesting that the orientation of TM2 is altered (Miura, S., and Karnik, S. S. (2002) J. Biol. Chem. 277, 24299-24305). Now we provide evidence that the TM2 helical movement plays a role in regulating the activated state of the AT(1) receptor, and this role may involve an interaction between TM2 and TM7. Alanine substitution of native Cys(296) in TM7 leads to increased accessibility of Cys(289) and diminished response to bound agonist. Both effects of the C296A mutation are suppressed when combined with F77A and N111G mutants. The TM7 conformation and the sensitivity of Cys(289) altered by C296A mutation are suppressed by the F77A mutation in TM2 to salvage function. We show that the F77A mutant alters orientation of both TM2 and TM7 but does not induce constitutive activity in suppressing the C296A mutant effects. Thus, interaction of TM2 and TM7 is important for transmembrane signal transduction in the AT(1) receptor.