Genetic interactions in yeast between Ypt GTPases and Arf guanine nucleotide exchangers

Two families of GTPases, Arfs and Ypt/rabs, are key regulators of vesicular transport. While Arf proteins are implicated in vesicle budding from the d onor compartment, Ypt/rab proteins are involved in the targeting of vesicle s to the acceptor compartment. Recently, we have shown a role for Ypt31/32p in exit from the yeast trans-Golgi, suggesting a possible function for Ypt /rab proteins in vesicle budding as well. Here we report the identification of a new member of the Sec7-domain family, SYT1, as a high-copy suppressor of a ypt31/32 mutation. Several proteins that belong to the Sec7-domain fa mily, including the yeast Gea1p, have recently been shown to stimulate nucl eotide exchange by Arf GTPases. Nucleotide exchange by Arf GTPases, the swi tch from the GDP- to the GTP-bound form, is thought to be crucial for their function. Sec7p itself has an important role in the yeast secretory pathwa y. However, its mechanism of action is not yet understood. We show that all members of the Sec7-domain family exhibit distinct genetic interactions wi th the YPT genes. Biochemical assays demonstrate that, although the homolog y between the members of the Sec7-domain family is relatively low (20-35%) and limited to a small domain, they all can act as guanine nucleotide excha nge factors (GEFs) for Arf proteins, but not for Ypt GTPases. The Sec7-doma in of Sec7p is sufficient for this activity. Interestingly, the Sec7 domain activity is inhibited by brefeldin A (BFA), a fungal metabolite that inhib its some of the Arf-GEFs, indicating that this domain is a target for BFA. These results demonstrate that the ability to act as Arf-GEFs is a general property of all Sec7-domain proteins in yeast. The genetic interactions obs erved between Xrf GEFs and Ypt GTPases suggest the existence of a Ypt-Arf G TPase cascade in the secretory pathway.