Regulation of Arf activation occurs via distinct mechanisms at early and late Golgi compartments.

At the Golgi complex, the biosynthetic sorting center of the cell, the Arf GTPases are responsible for coordinating vesicle formation. The Arf-GEFs activate Arf GTPases and are therefore the key molecular decision-makers for trafficking from the Golgi. In Saccharomyces cerevisiae , three conserved Arf-GEFs function at the Golgi: Sec7, Gea1, and Gea2. Our group has described the regulation of Sec7, the trans -Golgi Arf-GEF, through autoinhibition, positive feedback, dimerization, and interactions with a suite of small GTPases. However, we lack a clear understanding of the regulation of the early Golgi Arf-GEFs Gea1 and Gea2. Here we demonstrate that Gea1 and Gea2 prefer neutral over anionic membrane surfaces in vitro, consistent with their localization to the early Golgi. We illustrate a requirement for a critical mass of either Gea1 or Gea2 for cell growth under stress conditions. We show that the C-terminal domains of Gea1 and Gea2 toggle roles in the cytosol and at the membrane surface, preventing membrane binding in the absence of a recruiting interaction but promoting maximum catalytic activity once recruited. We also identify the small GTPase Ypt1 as a recruiter for Gea1 and Gea2. Our findings illuminate core regulatory mechanisms unique to the early Golgi Arf-GEFs.