Albert Eye Research Institute, Duke University, Durham, North Carolina.
Invest Ophthalmol Vis Sci. 2013 Nov 21;54(12):7725-33. doi: 10.1167/iovs.13-13281.
As the molecular mechanisms of vertebrate phototransduction became increasingly clear in the 1980s, a persistent problem was the discrepancy between the slow GTP hydrolysis catalyzed by the phototransduction G protein, transducin, and the much more rapid physiological recovery of photoreceptor cells from light stimuli. Beginning with a report published in 1989, a series of studies revealed that transducin GTPase activity could approach the rate needed to explain physiological recovery kinetics in the presence of one or more factors present in rod outer segment membranes. One by one, these factors were identified, beginning with PDEγ, the inhibitory subunit of the cGMP phosphodiesterase activated by transducin. There followed the discovery of the crucial role played by the regulator of G protein signaling, RGS9, a member of a ubiquitous family of GTPase-accelerating proteins, or GAPs, for heterotrimeric G proteins. Soon after, the G protein β isoform Gβ5 was identified as an obligate partner subunit, followed by the discovery or R9AP, a transmembrane protein that anchors the RGS9 GAP complex to the disk membrane, and is essential for the localization, stability, and activity of this complex in vivo. The physiological importance of all of the members of this complex was made clear first by knockout mouse models, and then by the discovery of a human visual defect, bradyopsia, caused by an inherited deficiency in one of the GAP components. Further insights have been gained by high-resolution crystal structures of subcomplexes, and by extensive mechanistic studies both in vitro and in animal models.
自 20 世纪 80 年代脊椎动物光转导的分子机制变得越来越清晰以来，一个持续存在的问题是光转导 G 蛋白转导蛋白催化的 GTP 水解与光感受器细胞从光刺激中快速恢复之间的差异。从 1989 年发表的一篇报告开始，一系列研究揭示了在杆状外节膜中存在一种或多种因子的情况下，转导蛋白 GTP 酶活性可以接近解释生理恢复动力学所需的速率。这些因子一个接一个地被识别出来，首先是 PDEγ，它是被转导蛋白激活的 cGMP 磷酸二酯酶的抑制亚基。随后发现了调节蛋白信号转导的 RGS9 所起的关键作用，它是普遍存在的 GTPase 加速蛋白或 GAPs 家族的一员，对异三聚体 G 蛋白起作用。不久之后，G 蛋白 β 亚基 Gβ5 被确定为必需的伴侣亚基，随后发现 R9AP 是一种跨膜蛋白，它将 RGS9 GAP 复合物锚定在盘膜上，对于该复合物在体内的定位、稳定性和活性是必不可少的。该复合物的所有成员的生理重要性首先通过敲除小鼠模型得到了明确，然后通过发现一种由 GAP 成分之一遗传缺陷引起的人类视觉缺陷——视力迟钝得到了明确。通过亚复合物的高分辨率晶体结构以及体外和动物模型的广泛机制研究，进一步深入了解了这一机制。
