The ADP Ribosylation Factor-Nucleotide Exchange Factors Gea1p and Gea2p Have Overlapping, but Not Redundant Functions in Retrograde Transport from the Golgi to the Endoplasmic Reticulum

Abstract

The activation of the small ras-like GTPase Arf1p requires the action of guanine nucleotide exchange factors. Four Arf1p guanine nucleotide exchange factors have been identified in yeast: Sec7p, Syt1p, Gea1p, and its homologue Gea2p. We identifiedGEA2 as a multicopy suppressor of asec21-3 temperature-sensitive mutant.SEC21 encodes the γ-subunit of coatomer, a heptameric protein complex that together with Arf1p forms the COPI coat.GEA1 and GEA2 have at least partially overlapping functions, because deletion of either gene results in no obvious phenotype, whereas the double null mutant is inviable. Conditional mutants defective in both GEA1 andGEA2 accumulate endoplasmic reticulum and Golgi membranes under restrictive conditions. The two genes do not serve completely overlapping functions because a Δgea1Δarf1 mutant is not more sickly than a Δarf1 strain, whereas Δgea2Δarf1 is inviable. Biochemical experiments revealed similar distributions and activities for the two proteins. Gea1p and Gea2p exist both in membrane-bound and in soluble forms. The membrane-bound forms, at least one of which, Gea2p, can be visualized on Golgi structures, are both required for vesicle budding and protein transport from the Golgi to the endoplasmic reticulum. In contrast, Sec7p, which is required for protein transport within the Golgi, is not required for retrograde protein trafficking.