CARBOXYL METHYLATION AND FARNESYLATION OF TRANSDUCIN GAMMA-SUBUNIT SYNERGISTICALLY ENHANCE ITS COUPLING WITH METARHODOPSIN-II

A heterotrimeric G-protein in vertebrate photoreceptor cells is called transducin (T-alpha-beta-gamma), whose gamma-subunit is a mixture of two components, T-gamma-1 and T-gamma-2. T-gamma-2 is S-farnesylated and partly carboxyl methylated at the C-terminal cysteine residue, whereas T-gamma-1 lacks the modified cysteine residue. To elucidate the physiological significance of the double modifications in T-gamma, we established a simple chromatographic procedure to isolate T-gamma-1, methylated T-gamma-2 and non-methylated T-gamma-2 on a reversed phase column. Taking advantage of the high and reproducible yield of T-gamma from the column, we analyzed the composition of T-gamma-subspecies in the T-alpha-T-beta-gamma-complex which did not bind with transducin-depleted rod outer segment membranes containing metarhodopsin II. The binding of T-alpha-T-beta-gamma with the membranes was shown to require the S-farnesylated cysteine residue of T-gamma, whose methylation further enhanced the binding. This synergistic effect was not evident when T-alpha was either absent or converted to the GTP-bound form which is known to dissociate from T-beta-gamma. Thus we concluded that a formation of the ternary complex, T-alpha-T-beta-gamma-metarhodopsin II, is enhanced by the farnesylation and methylation of T-gamma. This suggests that the double modifications provide most efficient signal transduction in photoreceptor cells.