The F₁-ATPase inhibitor Inh1 (IF1) affects suppression of mtDNA loss-lethality in Kluyveromyces lactis

Loss of mtDNA by the petite-negative yeast Kluyveromyces lactis is lethal (ρ^o-lethality). However, mutations in the α, β and γ subunits of F₁-ATPase can suppress lethality by increasing intramitochondrial hydrolysis of ATP. Increased hydrolysis of ATP can also occur on inactivation of Inh1, the natural inhibitor of F₁-ATPase. However, not all strains of K. lactis show suppression of ρ^o- lethality on inactivation of INH1. Genetic analysis indicates that one or more alleles of modifying factors are required for suppression. Papillae showing enhanced resistance to ethidium bromide (EB) in INH1 disruptants have mutations in the α, β and γ subunits of F₁-ATPase. Increased growth of double mutants on EB has been investigated by disruption of INH1 in previously characterized atp suppressor mutants. Inactivation of Inh1, with one exception, results in better growth on EB and increased F₁-ATPase activity, indicating that suppression of ρ^o-lethality is not due to atp mutations preventing Inh1 from interacting with the F₁- complex. By contrast, in suppressor mutants altered in Arg435 of the β subunit, disruption of INH1 did not change the kinetic properties of F ₁-ATPase or alter growth on EB. Consequently, Arg435 appears to be required for interaction of Inh1 with the β subunit. In a previous study, a mex1-1 allele was found to enhance mgi(atp) expression. In accord with results from double mutants, it has been found that mex1-1 is a frameshift mutation in INH1 causing inactivation of Inh1p.