ASC1/RAS2 Suppresses the Growth Defect on Glycerol Caused by the atp1–2 Mutation in the YeastSaccharomyces cerevisiae

Abstract To better define the regulatory role of the F1-ATPase α-subunit in the catalytic cycle of the ATP synthase complex, we isolated suppressors of mutations occurring inATP1, the gene for the α-subunit in Saccharomyces cerevisiae. First, two atp1 mutations (atp1–1 and atp1–2) were characterized that prevent the growth of yeast on non-fermentable carbon sources. Both mutants contained full-length F1α-subunit proteins in mitochondria, but in lower amounts than that in the parental strain. Both mutants exhibited barely measurable F1-ATPase activity. The primary mutations in atp1–1 andatp1–2 were identified as Thr383 → Ile and Gly291 → Asp, respectively. From recent structural data, position 383 lies within the catalytic site. Position 291 is located near the region affecting subunit-subunit interaction with the F1β-subunit. An unlinked suppressor gene,ASC1 (α-subunitcomplementing) of the atp1–2 mutation (Gly291 → Asp) restored the growth defect phenotype on glycerol, but did not suppress either atp1–1 or the deletion mutant Δatp1. Sequence analysis revealed thatASC1 was allelic with RAS2, a G-protein growth regulator. The introduction of ASC1/RAS2 into theatp1–2 mutant increased the F1-ATPase enzyme activity in this mutant when the transformant was grown on glycerol. The possible mechanisms of ASC1/RAS2 suppression ofatp1–2 are discussed; we suggest that RAS2 is part of the regulatory circuit involved in the control of F1-ATPase subunit levels in mitochondria.