Environmental response of yeast peroxisomes. Aspects of organelle assembly and degradation.

Nutritional changes can affect either the assembly or disassembly of yeast peroxisomes. In the past decade, insights regarding the molecular mechanisms of peroxisome assembly have been gained chiefly through the cloning of the PEX genes obtained by complementation of corresponding pex mutants in several yeast strains and Chinese hamster ovary cell lines. Depletion of these peroxins (proteins encoded by PEX genes) by deletion of the corresponding genes affects peroxisomal protein import, biogenesis, or proliferation. To complement these studies in the field, the authors undertook an investigation of the functions of a subset of Candida boidinii peroxisomal membrane proteins (PMPs), Pex11, Pmp47, and Pmp20, by analyzing strains of C. boidinii in which the genes encoding these proteins were deleted. The authors' studies show that Pex11p is involved in peroxisome proliferation; Pmp47 plays a role in the translocation, folding, or assembly of dihydroxyacetone synthase; and Pmp20 is probably involved in methanol metabolism. In contrast to the studies on peroxisome assembly, the molecular mechanisms of peroxisome degradation remain poorly understood. To shed light on this problem, the authors isolated Pichia pastoris mutants defective in peroxisome autophagy (pag mutants). A novel, double-fluorescence method used for the characterization of wild-type cells and of pag mutants enabled us to dissect the microautophagic degradation of peroxisomes into several distinct stages. These studies show that specific PAG gene products are involved in multiple steps of the process. Future cloning and characterization of the functions of PAG genes will reveal the molecular basis of peroxisome degradation.