Ogg S C, Barz W P, Walter P
Howard Hughes Medical Institute and Department of Biochemistry and Biophysics, University of California School of Medicine, San Francisco, California 94143-0448, USA.
J Cell Biol. 1998 Jul 27;142(2):341-54. doi: 10.1083/jcb.142.2.341.
The signal recognition particle and its receptor (SR) target nascent secretory proteins to the ER. SR is a heterodimeric ER membrane protein whose subunits, SRalpha and SRbeta, are both members of the GTPase superfamily. Here we characterize a 27-kD protein in Saccharomyces cerevisiae (encoded by SRP102) as a homologue of mammalian SRbeta. This notion is supported (a) by Srp102p's sequence similarity to SRbeta; (b) by its disposition as an ER membrane protein; (c) by its interaction with Srp101p, the yeast SRalpha homologue; and (d) by its role in SRP-dependent protein targeting in vivo. The GTP-binding site in Srp102p is surprisingly insensitive to single amino acid substitutions that inactivate other GTPases. Multiple mutations in the GTP-binding site, however, inactivate Srp102p. Loss of activity parallels a loss of affinity between Srp102p and Srp101p, indicating that the interaction between SR subunits is important for function. Deleting the transmembrane domain of Srp102p, the only known membrane anchor in SR, renders SR soluble in the cytosol, which unexpectedly does not significantly impair SR function. This result suggests that SR functions as a regulatory switch that needs to associate with the ER membrane only transiently through interactions with other components.
信号识别颗粒及其受体（SR）将新生分泌蛋白靶向内质网。SR是一种异二聚体内质网膜蛋白，其亚基SRα和SRβ都是GTP酶超家族的成员。在这里，我们将酿酒酵母中的一种27-kD蛋白（由SRP102编码）鉴定为哺乳动物SRβ的同源物。这一观点得到了以下几点支持：（a）Srp102p与SRβ的序列相似性；（b）它作为内质网膜蛋白的定位；（c）它与酵母SRα同源物Srp101p的相互作用；以及（d）它在体内SRP依赖性蛋白靶向中的作用。Srp102p中的GTP结合位点对使其他GTP酶失活的单个氨基酸取代出奇地不敏感。然而，GTP结合位点的多个突变会使Srp102p失活。活性丧失与Srp102p和Srp101p之间亲和力的丧失平行，表明SR亚基之间的相互作用对功能很重要。删除Srp102p的跨膜结构域（SR中唯一已知的膜锚）会使SR可溶于细胞质，出乎意料的是，这并没有显著损害SR的功能。这一结果表明，SR作为一种调节开关，只需要通过与其他成分的相互作用与内质网膜短暂结合。
