SELECTIVE AUTOPHAGY OF PEROXISOMES IN METHYLOTROPHIC YEASTS

The methylotrophic yeasts Pichia pastoris and Hansenula polymorpha res pond to a methanol substrate by synthesizing peroxisomal enzymes resul ting in the formation of large peroxisomes. When the carbon source was changed from methanol to glucose, we observed a rapid loss of peroxis omes. In this comparative study, we utilized biochemical and morpholog ical techniques to characterize the loss of peroxisomes in these yeast s. We used metabolic labeling and chase procedures to evaluate whether this loss was due to suppressed synthesis or enhanced degradation. Th e synthesis of alcohol oxidase was depressed 10-fold when cultures gro wn in methanol attained stationary growth. However, no further reducti on of synthesis was observed upon transfer of these cultures to glucos e medium. In stationary phase cultures maintained in methanol, two per oxisomal proteins, alcohol oxidase and dihydroxyacetone synthase, were degraded with a half-life of over 3 h. However, within 3 h of glucose repression, as much as 80% of the radiolabeled peroxisomal proteins w ere lost from both yeasts. This glucose-mediated degradative event app eared to be specific for peroxisomal proteins, since mitochondrial pro teins were stable. Ultrastructural examination of both yeasts revealed that glucose induced the sequestration of peroxisomes into the yeast vacuole, the presumed site of degradation. These results suggest that peroxisome loss during glucose repression is due to a selective, enhan ced degradation of whole peroxisomes by autophagic mechanisms.