Peterson J B
Department of Biochemistry, College of Agricultural and Life Sciences, University of Wisconsin-Madison 53706.
J Protozool. 1991 Sep-Oct;38(5):495-501. doi: 10.1111/j.1550-7408.1991.tb04823.x.
GTP-binding proteins act as molecular switches in a variety of membrane-associated processes, including secretion. One group of GTP-binding proteins, 20-30 kDa, is related to the product of the ras proto-oncogene. In Saccharomyces cerevisiae, ras-like GTP-binding proteins regulate vesicular traffic in secretion. The ciliate protist Paramecium tetraurelia contains secretory vesicles (trichocysts) whose protein contents are released by regulated exocytosis. Using [alpha-32P]GTP and an on-blot assay for GTP-binding, we detected at least seven GTP-binding proteins of low molecular mass (22-31 kDa) in extracts of Paramecium tetraurelia. Subcellular fractions contained characteristic subsets of these seven; cilia were enriched for the smallest (22 kDa). The pattern of GTP-binding proteins was altered in two mutants defective in the formation or discharge of trichocysts. Trichocysts isolated with their surrounding membranes intact contained two minor GTP-binding proteins (23.5 and 29 kDa) and one major GTP-binding protein (23 kDa) that were absent from demembranated trichocysts. This differential localization of GTP-binding proteins suggests functional specialization of specific GTP-binding proteins in ciliary motility and exocytosis.
GTP结合蛋白在包括分泌在内的多种膜相关过程中充当分子开关。一类分子量为20 - 30 kDa的GTP结合蛋白与原癌基因ras的产物相关。在酿酒酵母中，类ras GTP结合蛋白调节分泌过程中的囊泡运输。纤毛虫原生动物四膜虫含有分泌囊泡（刺丝泡），其蛋白质内容物通过受调控的胞吐作用释放。我们使用[α - 32P]GTP和一种用于检测GTP结合的印迹法，在四膜虫提取物中检测到至少七种低分子量（22 - 31 kDa）的GTP结合蛋白。亚细胞组分包含这七种蛋白的特征性子集；纤毛中富集了最小的（22 kDa）蛋白。在两个刺丝泡形成或释放有缺陷的突变体中，GTP结合蛋白的模式发生了改变。完整地分离出的带有周围膜的刺丝泡含有两种次要的GTP结合蛋白（23.5和29 kDa）和一种主要的GTP结合蛋白（23 kDa），而脱膜的刺丝泡中则没有这些蛋白。GTP结合蛋白的这种差异定位表明特定的GTP结合蛋白在纤毛运动和胞吐作用中具有功能特异性。
