IDENTIFICATION OF REGULATORS FOR YPT1 GTPASE NUCLEOTIDE CYCLING

Small GTPases of the Ypt/Rab family are involved in the regulation of vesicular transport. Cycling between the GDP- and GTP-bound forms and the accessory proteins that regulate this cycling are thought to be cr ucial for Ypt/Rab function. Guanine nucleotide exchange factors (GEFs) stimulate both GDP loss and GTP uptake, and GTPase-activating protein s (GAPs) stimulate GTP hydrolysis. Little is known about GEFs and GAPs for Ypt/Rab proteins. In this article we report the identification an d initial characterization of two factors that regulate nucleotide cyc ling by Ypt1p, which is essential for the first two steps of the yeast secretory pathway. The Ypt1p-GEF stimulates GDP release and GTP uptak e at least 10-fold and is specific for Ypt1p. Partially purified Ypt1p -GEF can rescue the inhibition caused by the dominant-negative Ypt1p-D 124N mutant of in vitro endoplasmic reticulum-to-Golgi transport. This mutant probably blocks transport by inhibiting the GEF, suggesting th at we have identified the physiological GEF for Ypt1p. The Ypt1p-GAP s timulates GTP hydrolysis by Ypt1p up to 54-fold, has a higher affinity for the GTP-bound form of Ypt1p than for the GDP-bound form, and is s pecific to a subgroup of exocytic Ypt proteins. The Ypt1p-GAP activity is not affected by deletion of two genes that encode known Ypt GAPs, GYP7 and GYP1, nor is it influenced by mutations in SEC18, SEC17, or S EC22, genes whose products are involved in vesicle fusion. The GEF and GAP activities for Ypt1p localize to particulate cellular fractions. However, contrary to the predictions of current models, the GEF activi ty localizes to the fraction that functions as the acceptor in an endo plasmic reticulum-to-Golgi transport assay, whereas the GAP activity c ofractionates with markers for the donor. On the basis of our current and previous results, we propose a new model for the role of Ypt/Rab n ucleotide cycling and the factors that regulate this process.