Arf proteins are ubiquitous, eukaryotic regulators of virtually every step of vesicular membrane traffic. ADP-ribosylation factors are essential in yeast and the lethality resulting from either overexpression or underexpression (deletion) of Arf genes has previously been ascribed to dysregulation of the secretory process. We have identified a family of four genes (Suppressors of Arf ts, SAT) as high copy suppressors of a loss of function allele of ARF1 (arf1-3). Those proteins with SAT activity were found to contain a minimal consensus motif, including a C2C2H2 cluster with a novel and specific spacing. Genetic interactions between members of this family and with ARF1 are consistent with each sharing a common cellular pathway. Included in this family is Gcs1, a protein previously described (Poon, P. P., Wang, X., Rotman, M., Huber, I., Cukierman, E., Cassel, D., Singer, R. A., and Johnston, G. C. (1996) Proc. Natl. Acad. Sci. U. S. A. 93, 10074-10077) to possess Arf GTPase-activating protein (GAP) activity, demonstrating a direct interaction between Arf and at least one of these suppressors. The suppression of the loss of Arf function by overexpression of Gcs1 and demonstration of direct, preferential binding of Gcs1 to the activated form of Arf (Arf.GTP) lead us to conclude that the biological role of Gcs1 is as an effector of the essential function of Arf in mitotic growth, rather than a down-regulator as implied by the biochemical (Arf GAP) activity. Suppression of the growth defect of arf1(-3) cells was observed under conditions that did not alter the secretory defect associated with arf1(-) mutation, indicating that the essential role of Arf in eukaryotes can be distinguished from role(s) in the secretory pathway and appear to employ distinct pathways and effectors.