Impairment of Peroxisome Degradation in Pichia methanolica Mutants Defective in Acetyl‐CoA Synthetase or Isocitrate Lyase

Single recessive mutations of the methylotrophic yeast Pichia methanolica acs1 , acs2 , acs3 and icl1 affecting acetyl‐CoA synthetase and isocitrate lyase, and growth on ethanol as sole carbon and energy source, caused a defect in autophagic peroxisome degradation during exposure of methanol‐grown cells to ethanol. As a control, a mutation in mdd1 , which resulted in a defect of the ‘malic’ enzyme and also prevented ethanol utilization, did not prevent peroxisome degradation. Peroxisome degradation in glucose medium was unimpaired in all strains tested. Addition of ethanol to methanol‐grown cells of acs1 , acs2 , acs3 and icl1 mutants led to an increase in average vacuole size. Thickening of peroxisomal membranes and tight contacts between groups of peroxisomes and vacuoles were rarely observed. These processes proceeded much more slowly than in wild‐type or mdd1 mutant cells incubated under similar conditions. No peroxisomal remnants were observed inside vacuoles in the cells of acs1 , acs2 , acs3 and icl1 mutants after prolonged cultivation in ethanol medium. We hypothesize that the acs and icl mutants are defective in synthesis of the true effector—presumably glyoxylate—of peroxisome degradation in ethanol medium. Lack of the effector suspends peroxisome degradation at an early stage, namely signal transduction or peroxisome/vacuole recognition. Finally, these defects in peroxisome degradation resulted in mutant cells retaining high levels of alcohol oxidase which further led to increased levels of acetaldehyde accumulation upon incubation of mutant cells with ethanol. © 1997 by John Wiley & Sons, Ltd.