Distinct and specific GAP activities in rat pancreas on the yeast GTP-binding proteins Ypt1 and Sec4

• Published in: FEBS letters 1992-08, Vol.309 (1), p.5-9
• Main Authors: Jena, B.P., Brennwald, P., Garrett, M.D., Novick, P., Jamieson, J.D.
• Format: Article
• Language: English
• Subjects: 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid; bovine serum albumin; BSA; dithiothreitol; DTT; HEPES; Pancreatic Sec4-GAP and Ypt1-GAP; PEI-TLC; phenylmethylsulfonyl fluoride; PMSF; polyethyleneimine cellulose thin layer chromatography; SBTI; soybean trypsin inhibitor; Yeast GTP-binding protein; Ypt1 and Sec4
• ISSN: 0014-5793; 1873-3468
• DOI: 10.1016/0014-5793(92)80727-X

Previous studies have demonstrated that Sec4, a 23.5 kDa guanine nucleotide-binding protein of the ras superfamily is required for exocytosis in the budding yeast Saccharomyees cerevisae, Ypt1, another ras-like 23 kDa guanine nucleotide-binding protein in yeast has been found to be involved in ER-Golgi transport. A mammalian homologue of Ypt1 called rabl has also been identified. Recent studies using purified Sec4 protein have identified a component of yeast lysate that specifically stimulates the hydrolysis of GTP bound to Sec4. In the present study, purified recombinant Sec4 and Ypt1 proteins expressed in E. coli have been used as substrates to determine if GTPase activating proteins (GAPs) directed toward these proteins are present in rat pancreas. Our studies showed that 65% of Sec4-GAP activity was associated with the 150,000 × g pancreatic particulate fraction with approximately 35% being found in the cytosol. On the other hand, more than 95% of Ypt1-GAP activity was found to associate with the particulate fraction. Sec4 and Ypt1 competition assays further demonstrated the specificity of the Sec4 and Ypt1 GAPs. The results from the present study suggest the presence of a distinct GAP in the pancreas that interacts with Sec4, and another that interacts with Ypt1.