Selective autophagy of peroxisomes in methylotrophic yeasts

The methylotrophic yeasts Pichia pastoris and Hansenula polymorpha respond to a methanol substrate by synthesizing peroxisomal enzymes resulting in the formation of large peroxisomes. When the carbon source was changed from methanol to glucose, we observed a rapid loss of peroxisomes. In this comparative study, we utilized biochemical and morphological techniques to characterize the loss of peroxisomes in these yeasts. We used metabolic labeling and chase procedures to evaluate whether this loss was due to suppressed synthesis or enhanced degradation. The synthesis of alcohol oxidase was depressed 10-fold when cultures grown in methanol attained stationary growth. However, no further reduction of synthesis was observed upon transfer of these cultures to glucose medium. In stationary phase cultures maintained in methanol, two peroxisomal 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 were lost from both yeasts. This glucose-mediated degradative event appeared to be specific for peroxisomal proteins, since mitochondrial proteins 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, enhanced degradation of whole peroxisomes by autophagic mechanisms.