Diffusible ligand all-trans-retinal activates opsin via a palmitoylation-dependent mechanism

In rhodopsin's function as it photoreceptor, 11-cis-retinal is covalently b ound to Lys(296) via a protonated Schiff base. 11-cis/all-trans photoisomer ization and relaxation through intermediates lead to the metarhodopsin II p hotoproduct, which couples to transducin (G(t)), Here we have analyzed a di fferent signaling state that arises from noncovalent binding of all-trans r etinal (atr) to the aporeceptor opsin and enhances the very low opsin activ ity by several orders of magnitude. Like with metarhodopsin II, coupling of G(t) to opsin-atr is sensitive to competition by synthetic peptides from t he COOH termini of both G(t)alpha and G(t)gamma. However, atr does not comp ete with 11-cis-retinal incorporation into the Lys(296) binding site and fo rmation of the light-sensitive pigment. Blue light illumination fails to ph otorevert opsin-atr to the ground state. Thus noncovalently bound atr has n o access to the light-dependent binding site and reaction pathway. Moreover , in contrast to light-dependent signaling, removal of the palmitoyl anchor s at Cys(322) and Cys(323) in the rhodopsin COOH terminus impairs the atr-s timulated activity. Repahmitoylation by autoacylation with palmitoyl-coenzy me A restores most of the original activity. We hypothesize that the palmit oyl moieties are part of a second binding pocket for the chromophore, media ting hydrophobic interactions that can activate a large part of the catalyt ic receptor/G-protein interface.