In many rhodopsin-like G-protein-coupled receptors, agonist binding to a cluster of aromatic residues in TM6 may promote receptor activation by altering the configuration of the TM6 Pro-kink and by the subsequent movement of the cytoplasmic end of TM6 away from TM3. We hypothesized that the highly conserved Cys 6.47 , in the vicinity of the conserved Pro 6.50 , modulates the configuration of the aromatic cluster and the TM6 Prokink through specific interactions in its different rotamer

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... tions. In the ␤ 2 adrenergic receptor, mutation of Cys 6.47 to Thr, which in an ␣-helix has a different rotamer distribution from Cys and Ser, produced a constitutively active receptor, whereas the Ser mutant was similar to wild-type receptor. Use of the biased Monte Carlo technique of Conformational Memories showed that the rotamer changes among Cys/Ser/Thr 6.47 , Trp 6.48 , and Phe 6.52 are highly correlated, representing a rotamer "toggle switch" that may modulate the TM6 Pro-kink. Differential modulation of the accessibility of Cys 6.47 and an engineered Cys 6.52 in wild type and a constitutively active background provides experimental support for the association of this rotamer switch with receptor activation.