Dirac-Svejstrup A B, Sumizawa T, Pfeffer S R
Department of Biochemistry, Stanford University School of Medicine, CA 94305-5307, USA.
EMBO J. 1997 Feb 3;16(3):465-72. doi: 10.1093/emboj/16.3.465.
Prenylated Rab GTPases occur in the cytosol in their GDP-bound conformations bound to a cytosolic protein termed GDP-dissociation inhibitor (GDI). Rab-GDI complexes represent a pool of active, recycling Rab proteins that can deliver Rabs to specific and distinct membrane-bound compartments. Rab delivery to cellular membranes involves release of GDI, and the membrane-associated Rab protein then exchanges its bound GDP for GTP. We report here the identification of a novel, membrane-associated protein factor that can release prenylated Rab proteins from GDI. This GDI-displacement factor (GDF) is not a guanine nucleotide exchange factor because it did not influence the intrinsic rates of nucleotide exchange by Rabs 5, 7 or 9. Rather, GDF caused the release of each of these endosomal Rabs from GDI, permitting them to exchange nucleotide at their intrinsic rates. GDF displayed the greatest catalytic rate enhancement on Rab9-GDI complexes. However, catalytic rate enhancement paralleled the potency of GDI in blocking nucleotide exchange: GDI was shown to be most potent in blocking nucleotide exchange by Rab9. The failure of GDF to act on Rab1-GDI complexes suggests that it may be specific for endosomal Rab proteins. This novel, membrane-associated activity may be part of the machinery used to localize Rabs to their correct intracellular compartments.
异戊二烯化的Rab GTP酶以其与GDP结合的构象存在于胞质溶胶中，与一种称为GDP解离抑制剂（GDI）的胞质蛋白结合。Rab - GDI复合物代表了一组活跃的、可循环利用的Rab蛋白，它们能够将Rab蛋白递送至特定且不同的膜结合区室。Rab蛋白向细胞膜的递送涉及GDI的释放，然后膜相关的Rab蛋白将其结合的GDP交换为GTP。我们在此报告鉴定出一种新型的膜相关蛋白因子，它能够从GDI中释放异戊二烯化的Rab蛋白。这种GDI置换因子（GDF）不是鸟嘌呤核苷酸交换因子，因为它不影响Rab5、Rab7或Rab9的核苷酸交换内在速率。相反，GDF导致这些内体Rab蛋白中的每一种从GDI中释放出来，使它们能够以其内在速率交换核苷酸。GDF对Rab9 - GDI复合物表现出最大的催化速率增强作用。然而，催化速率增强与GDI阻断核苷酸交换的效力平行：已表明GDI在阻断Rab9的核苷酸交换方面最有效。GDF不能作用于Rab1 - GDI复合物，这表明它可能对内体Rab蛋白具有特异性。这种新型的膜相关活性可能是用于将Rab蛋白定位到其正确的细胞内区室的机制的一部分。
