ASC1/RAS2 suppresses the growth defect on glycerol caused by the atp1-2 mutation in the yeast Saccharomyces cerevisiae

To better define the regulatory role of the F-1-ATPase Lu-subunit in the ca talytic cycle of the ATP synthase complex, we isolated suppressors of mutat ions occurring in ATP1, the gene for the alpha-subunit in Saccharomyces cer evisiae. First, two atp1 mutations (atp1-1 and atp1-2) mere characterized t hat prevent the growth of yeast on non-fermentable carbon sources. Both mut ants contained full-length F-1 alpha-subunit proteins in mitochondria, but in lower amounts than that in the parental strain. Both mutants exhibited b arely measurable F-1- ATPase activity. The primary mutations in atp1-1 and afp1-2 were identified as Thr(383)-->Ile and Gly(291)-->Asp, respectively. From recent structural data, position 383 lies within the catalytic site. P osition 291 is located near the region affecting subunit-subunit interactio n with the F(1)beta-subunit. An unlinked suppressor gene, ASC1 (alpha-subun it complementing) of the atp1-2 mutation (Gly(291) --> Asp) restored the gr owth defect phenotype on glycerol, but did not suppress either atp1-1 or th e deletion mutant Delta atp1. Sequence analysis revealed that ASC1 was alle lic with RAS2, a G-protein growth regulator. The introduction of ASC1/RAS2 into the atp1-2 mutant increased the F-1-ATPase enzyme activity in this mut ant when the transformant was grown on glycerol, The possible mechanisms of ASC1/RAS2 suppression of atp1-2 are discussed; we suggest that RAS2 is par t of the regulatory circuit involved ill the control of F-1-ATPase subunit levels in mitochondria.