Two families of GTPases, Arf and Ypt/rab, are key regulators
of vesicular transport. While Arf proteins are implicated in vesicle
budding from the donor compartment, Ypt/rab proteins are involved in the
targeting of vesicles to the acceptor compartment. Recently, we have
shown a role for Ypt31/32p in exit from the yeast trans-Golgi,
suggesting a 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 family, including the yeast
Gea1p, have been shown recently to stimulate nucleotide exchange by Arf
GTPases. Sec7p itself has an important role in the yeast secretory
pathway. However, its mechanism of action is not yet understood. We show
that SEC7 also exhibit reciprocal genetic interactions with the YPT31/32
genes. Biochemical assays demonstrate the ability of Sec7p and Syt1p to
promote nucleotide exchange on Arf proteins, but not on Ypt GTPases. The
Sec7-domain of Sec7p is sufficient for this activity. These results
demonstrate that the ability to act as Arf exchangers is a general
property of all Sec7-domain proteins in yeast. The genetic interactions
observed between Arf exchangers and Ypt31/32p suggest the existence of a
Ypt-Arf GTPase cascade in the secretory pathway. In addition, the
genetic interaction of Arf exchangers with Ypt31/32p supports the idea
of a role for Ypt GTPases in vesicle budding.
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