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.