Carboxyl methylation and farnesylation of transducin gamma-subunit synergistically enhance its coupling with metarhodopsin II.

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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.

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