EFFECTS OF CARBOXYL METHYLATION OF PHOTORECEPTOR G-PROTEIN GAMMA-SUBUNIT IN VISUAL TRANSDUCTION

G protein gamma-subunits are isoprenylated and carboxyl-methylated at the C-terminal cysteine, which is indispensable for the function of ph otoreceptor G protein transducin (T alpha beta gamma). However, the ph ysiological role of the methylation and its reversibility have been un clear, Here we isolated methylated and non-methylated forms of farnesy lated T beta gamma, and demonstrated that the methylation remarkably f acilitates not only the membrane association of T beta gamma but also the subunit interaction between T alpha and T beta gamma. Consequently , the functional coupling of transducin with light-activated receptor, metarhodopsin II, was stabilized by the methylation, resulting in acc eleration of GTP gamma S (guanosine 5'-3-0-(thio) triphosphate) bindin g to T(alpha. An examination of the reversibility of the methylation s uggested that T gamma is kept fully methylated in rod outer segments. These observations indicate that the methylation of T gamma plays an i mportant role in the most efficient photon-signal transduction process in rod cells.