Functional interaction of transmembrane helices 3 and 6 in rhodopsin. Replacement of phenylalanine 261 by alanine causes reversion of phenotype of a glycine 121 replacement mutant.
Replacement of a highly conserved glycine residue on transmembrane (TM) helix 3 of bovine rhodopsin (Gly121) by amino acid residues with larger side chains causes a progressive blue-shift in the lambdamax value of the pigment, a decrease in thermal stability, and an increase in reactivity with hydroxylamine. In addition, mutation of Gly121 causes a relative reversal in the selectivity of opsin for 11-cis-retinal over all-trans-retinal. It was suggested that Gly121 plays an important role in defining the 11-cis-retinal binding pocket of rhodopsin (Han, M., Lin, S. W., Smith, S. O., and Sakmar, T. P. (1996) J. Biol. Chem. 271, 32330-32336). Here, we combined the mutant opsin G121L with second site replacements of four different amino acid residues on TM helix 6: Met257, Val258, Phe261, or Trp265. We show that the loss of function phenotypes of the G121L mutant described above can be partially reverted specifically by the mutation of Phe261, a residue highly conserved in all G protein-coupled receptors. For example, the double-replacement mutant G121L/F261A has spectral, chromophore-binding, and transducin-activating properties intermediate between those of G121L and rhodopsin. This rescue of the G121L defects did not occur with the other second site mutations tested. We conclude that specific portions of TM helices 3 and 6, which include Gly121 and Phe261, respectively, define the chromophore-binding pocket in rhodopsin. Finally, the results are placed in the context of a molecular graphics model of the TM domain of rhodopsin, which includes the retinal-binding pocket.