Conformations of the active and inactive states of opsin

The signaling state metarhodopsin II of the visual pigment rhodopsin decays to the apoprotein opsin and all-trans retinal, which are then regenerated to rhodopsin by the visual cycle. Opsin is known to have at neutral pH only a small residual constitutive activity toward its G protein transducin, wh ich is thought to play a considerable role in light adaptation (bleaching d esensitization). In this study we show with Fourier-transform infrared spec troscopy that after metarhodopsin II decay, opsin exists in two conformatio nal states that are in a pH-dependent equilibrium at 30 degreesC with a pK of 4.1 in the presence of hydroxylamine scavenging the endogenous all-trans retinal. Despite the lack of the native agonist in its binding pocket, the low pH opsin conformation is very similar to that of metarhodopsin II and is likewise stabilized by peptides derived from rhodopsin's cognate G prote in, transducin. The high pH form, on the other hand, has some conformationa l similarity to the inactive metarhodopsin I state. We therefore conclude t hat the opsin apoprotein displays intrinsic conformational states that are merely modulated by bound all-trans retinal.