Pfeuffer T, Helmreich E J
Department of Physiological Chemistry, University of Würzburg, Federal Republic of Germany.
Curr Top Cell Regul. 1988;29:129-216. doi: 10.1016/b978-0-12-152829-4.50006-9.
Information available at present documents the existence of three well-defined classes of guanine nucleotide binding proteins functioning as signal transducers: Gs and Gi which stimulate and inhibit adenylate cyclase, respectively, and transducin which transmits and amplifies the signal from light-activated rhodopsin to cGMP-dependent phosphodiesterase in ROS membranes. Go is a fourth member of this family. Its function is the least known among GTP binding signal transducing proteins. The family of G proteins has a number of properties in common. All are heterotrimers consisting of three subunits, alpha, beta, and gamma. Each of the subunits may be heterogeneous depending on species and tissue of origin and may be posttranslationally modified covalently. The alpha subunits vary in size from 39 to 52 kDa. The sequences for Gs alpha and transducin alpha have 42% overall homology and those of Gi alpha and Gs alpha 43%, whereas those of Gi alpha and transducin alpha have a higher degree (68%) of homology. All alpha subunits bind guanine nucleotides and are ADP-ribosylated by either pertussis toxin (Gi, transducin, Go) or cholera toxin (Gs, Gi, transducin). Thus, transducin and Gi, which have the highest degree of sequence homology, are also ADP-ribosylated by both toxins. The beta subunits have molecular weights of 36 and 35 kDa, respectively. While Gs, Gi, and Go contain a mixture of both, transducin contains only the larger (36-kDa) beta-polypeptide. The relationship of the 36- and the 35-kDa beta subunits is not defined. Although the complete sequence of the 36-kDa beta subunit of transducin has been deduced from the cDNA sequence, complete sequences of other beta subunits are not yet available so that detailed comparisons cannot be made at present. However, the proteolytic profiles of each class of the beta subunits of different G proteins are indistinguishable. The gamma subunit of bovine transducin has been completely sequenced. It has a Mr of 8400. Again complete sequences of other gamma subunits are not yet available. While the gamma subunits of Gs, Gi, and Go have identical electrophoretic mobility in SDS gels, they differ significantly in this respect from the gamma subunit of transducin. Moreover, crossover experiments point to functional differences between gamma subunits from G protein and transducin complexes. In addition, a role for beta, gamma in anchoring guanine nucleotide binding proteins to membranes has been postulated.(ABSTRACT TRUNCATED AT 400 WORDS)
Gs和Gi，分别刺激和抑制腺苷酸环化酶；以及转导素，它将光激活的视紫红质的信号传递并放大至视网膜杆状细胞外节（ROS）膜中的环鸟苷酸依赖性磷酸二酯酶。Go是该家族的第四个成员。在GTP结合信号转导蛋白中，其功能了解得最少。G蛋白家族有许多共同特性。所有成员均为异源三聚体，由α、β和γ三个亚基组成。每个亚基可能因物种和来源组织而异，并且可能在翻译后进行共价修饰。α亚基的大小在39至52 kDa之间变化。Gsα和转导素α的序列总体同源性为42%，Giα和Gsα的同源性为43%，而Giα和转导素α的同源性更高（68%）。所有α亚基都结合鸟嘌呤核苷酸，并被百日咳毒素（Gi、转导素、Go）或霍乱毒素（Gs、Gi、转导素）进行ADP核糖基化。因此，序列同源性最高的转导素和Gi也都被这两种毒素进行ADP核糖基化。β亚基的分子量分别为36 kDa和35 kDa。虽然Gs、Gi和Go含有两者的混合物，但转导素只含有较大的（36 kDa）β多肽。36 kDa和35 kDaβ亚基之间的关系尚不清楚。尽管转导素36 kDaβ亚基的完整序列已从cDNA序列推导得出，但其他β亚基的完整序列尚未可得，因此目前无法进行详细比较。然而，不同G蛋白的每类β亚基的蛋白水解图谱无法区分。牛转导素的γ亚基已完全测序。其相对分子质量为8400。同样，其他γ亚基的完整序列也尚未可得。虽然Gs、Gi和Go的γ亚基在SDS凝胶中具有相同的电泳迁移率，但在这方面它们与转导素的γ亚基有显著差异。此外，交叉实验表明G蛋白和转导素复合物的γ亚基之间存在功能差异。另外已推测β、γ在将鸟嘌呤核苷酸结合蛋白锚定到膜上起作用。（摘要截短于400字）
