Ohguro H, Fukada Y, Takao T, Shimonishi Y, Yoshizawa T, Akino T
Department of Ophthalmology, Sapporo Medical College, Japan.
EMBO J. 1991 Dec;10(12):3669-74. doi: 10.1002/j.1460-2075.1991.tb04934.x.
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.
脊椎动物光感受器细胞中的一种异源三聚体G蛋白称为转导素（Tαβγ），其γ亚基由Tγ-1和Tγ-2两种成分组成。Tγ-2在C末端半胱氨酸残基处进行了S-法尼基化和部分羧基甲基化，而Tγ-1缺乏修饰的半胱氨酸残基。为了阐明Tγ双重修饰的生理意义，我们建立了一种简单的色谱方法，在反相柱上分离Tγ-1、甲基化的Tγ-2和未甲基化的Tγ-2。利用从柱中获得的高产率且可重复的Tγ，我们分析了Tα-Tβγ复合物中Tγ亚基的组成，该复合物不与含有视紫红质II的转导素耗尽的视杆外段膜结合。结果表明，Tα-Tβγ与膜的结合需要Tγ的S-法尼基化半胱氨酸残基，其甲基化进一步增强了结合。当Tα不存在或转化为已知会与Tβγ解离的GTP结合形式时，这种协同效应并不明显。因此我们得出结论，Tγ的法尼基化和甲基化增强了三元复合物Tα-Tβγ-视紫红质II的形成。这表明双重修饰在光感受器细胞中提供了最有效的信号转导。
