ADP-ribosylation factors do not activate yeast phospholipase Ds but are required for sporulation

ADP-ribosylation factor (ARF) proteins in Saccharomyces cerevisiae are encoded by two genes, ARF1 and ARF2. The addition of the c-myc epitope at the C terminus of Arf1 resulted in a mutant (arf1-myc arf2) that supported vegetative growth and rescued cells from supersensitivity to fluoride, but homozygous diploids failed to sporulate. arf1-myc arf2 mutants completed both meiotic divisions but were unable to form spores. The SPO14 gene encodes a phospholipase D (PLD), whose activity is essential for mediating the formation of the prospore membrane, a prerequisite event for spore formation. Spo14 localized normally to the developing prospore membrane in arf1-myc arf2 mutants; however, the synthesis of the membrane was attenuated. This was not a consequence of reduced PLD catalytic activity, because the enzymatic activity of Spol4 was unaffected in meiotic arf1-myc arf2 mutants. Although potent activators of mammalian PLD1, Arf1 proteins did not influence the catalytic activities of either Spol4 or ScPld2 a second yeast PLD. These results demonstrate that ARF1 is required for sporulation, and the mitotic and meiotic functions of Arf proteins are not mediated by the activation of any known yeast PLD activities. The implications of these results are discussed with respect to current models of Arf signaling.