IMPAIRMENT OF PEROXISOME DEGRADATION IN PICHIA-METHANOLICA MUTANTS DEFECTIVE IN ACETYL-COA SYNTHETASE OR ISOCITRATE LYASE

Single recessive mutations of the methylotrophic yeast Pichia methanol ica acs1, acs2, acs3 and icl1 affecting acetyl-CoA synthetase and isoc itrate lyase, and growth on ethanol as sole carbon and energy source, caused a defect in autophagic peroxisome degradation during exposure o f methanol-grown cells to ethanol. As a control, a mutation in mddl, w hich resulted in a defect of the 'malic' enzyme and also prevented eth anol utilization, did not prevent peroxisome degradation. Peroxisome d egradation in glucose medium was unimpaired in all strains tested. Add ition of ethanol to methanol-grown cells of acs1, acs2, acs3 and icl1 mutants led to an increase in average vacuole size. Thickening of pero xisomal membranes and tight contacts between groups of peroxisomes and vacuoles were rarely observed. These processes proceeded much more sl owly than in wild-type or mddl mutant cells incubated under similar co nditions. No peroxisomal remnants were observed inside vacuoles in the cells of acs1, acs2, acs3 and icl1 mutants after-prolonged cultivatio n in ethanol medium. We hypothesize that the ncs and icl mutants are d efective in synthesis of the true effector-presumably glyoxylate-of pe roxisome degradation in ethanol medium. Lack of the effector suspends peroxisome degradation at an early stage, namely signal transduction o r 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 accumul ation upon incubation of mutant cells with ethanol. (C) 1997 John Wile y & Sons, Ltd.