A Screen for Nigericin-Resistant Yeast Mutants Revealed Genes Controlling Mitochondrial Volume and Mitochondrial Cation Homeostasis

Abstract

Little is known about the regulation of ion transport across the inner mitochondrial membrane in Saccharomyces cerevisiae. To approach this problem, we devised a screening procedure for facilitating the identification of proteins involved in mitochondrial ion homeostasis. Taking advantage of the growth inhibition of yeast cells by electroneutral K 1 /H 1 ionophore nigericin, we screened for genetic mutations that would render cells tolerant to this drug when grown on a nonfermentable carbon source and identified several candidate genes including MDM31, MDM32, NDI1, YMR088C (VBA1), CSR2, RSA1, YLR024C, and YNL136W (EAF7). Direct examination of intact cells by electron microscopy indicated that mutants lacking MDM31 and/or MDM32 genes contain dramatically enlarged, spherical mitochondria and that these morphological abnormalities can be alleviated by nigericin. Mitochondria isolated from the Dmdm31 and Dmdm32 mutants exhibited limited swelling in an isotonic solution of potassium acetate even in the presence of an exogenous K 1 /H 1 antiport. In addition, growth of the mutants was inhibited on ethanol-containing media in the presence of high concentrations of salts (KCl, NaCl, or MgSO 4 ) and their mitochondria exhibited two-(Dmdm31 and Dmdm32) to threefold (Dmdm31Dmdm32) elevation in magnesium content. Taken together, these data indicate that Mdm31p and Mdm32p control mitochondrial morphology through regulation of mitochondrial cation homeostasis and the maintenance of proper matrix osmolarity.

Mcp1 and Mcp2, two novel proteins involved in mitochondrial lipid homeostasis

“…Mdm31 was previously shown to be involved in mitochondrial morphogenesis, inheritance (Dimmer et al, 2005) and ion homeostasis (Kucejova et al, 2005). However, the precise molecular function of this protein is currently unknown.…”

The Saccharomyces cerevisiae Nrd1-Nab3 Transcription Termination Pathway Acts in Opposition to Ras Signaling and Mediates Response to Nutrient Depletion

“…Previous deletion studies of MDM31 indicated that Mdm31p and Mdm32p control mitochondrial morphology (22,42) and are necessary for the proper inheritance and organization of mitochondrial DNA (22). The overexpression of MDM31 has not been described previously, so it is unclear whether the overexpression of MDM31 alone could cause the overabundance of mitochondria found in nrd1 mutants.…”

Mitochondria as protean organelles: membrane processes that influence mitochondrial shape in yeast

“…Analysis of such mutants could provide new insights into cellular ion homeostasis, as well as other related processes. In line with this assumption, we identified Δmdm31 and Δmdm32 among disruption mutants resistant to nigericin and their giant spherical mitochondria were shown to be swollen due to disturbed ion homeostasis (Kucejova et al 2005 (Kucejova et al 2005). Our screening of UV-induced nigericin and valinomycin resistant mutants yielded four independent clones, val R -1,2 and val/nig R -1,2, with disturbed mitochondrial morphology, one of them having also vacuolar defects (Petrezselyova et al 2007).…”

“…The deletion of either one of the MDM31 and MDM32 genes is synthetically lethal with deletion of either one of the MMM1, MDM12, MDM10, and MDM34 genes (Dimmer et al 2005). In addition, in Δmdm31 and Δmdm32 mutants, mitochondria remain attached to actin filaments, but the colocalization of the MMM complex with mitochondrial DNA (mtDNA) is rare and the nucleoids are misshapen, aggregated and destabilized (Dimmer et al 2005;Kucejova et al 2005) similarly as in Δmmm1 (Hobbs et al 2001) and Δmdm34 (Youngman et al 2004). Therefore, Mdm31p and Mdm32p were proposed to play a role in nucleoid anchoring via cooperation with the MMM complex.…”