MUTANTS OF THE YEAST YARROWIA-LIPOLYTICA DEFECTIVE IN PROTEIN EXIT FROM THE ENDOPLASMIC-RETICULUM ARE ALSO DEFECTIVE IN PEROXISOME BIOGENESIS

Mutations in the SEC238 and SRP54 genes of the yeast Yarrowia lipolyti ca not only cause temperature-sensitive defects in the exit of the pre cursor form of alkaline extracellular protease and of other secretory proteins from the endoplasmic reticulum and in protein secretion but a lso lead to temperature-sensitive growth in oleic acid containing medi um, the metabolism of which requires the assembly of functionally inta ct peroxisomes. The sec238A and srp54KO mutations at the restrictive t emperature significantly reduce the size and number of peroxisomes, af fect the import of peroxisomal matrix and membrane proteins into the o rganelle, and significantly delay, but do not prevent, the exit of two peroxisomal membrane proteins, Pex2p and Pex16p, from the endoplasmic reticulum en route to the peroxisomal membrane. Mutations in the PEX1 and PEX6 genes, which encode members of the AAA family of N-ethylmale imide-sensitive fusion protein-like ATPases, not only affect the exit of precursor forms of secretory proteins from the endoplasmic reticulu m but also prevent the exit of the peroxisomal membrane proteins Pex2p and Pex16p from the endoplasmic reticulum and cause the accumulation of an extensive network of endoplasmic reticulum membranes. None of th e peroxisomal matrix proteins tested associated with the endoplasmic r eticulum in sec238A, srp54KO, pex1-1, and pex6KO mutant cells. Our dat a provide evidence that the endoplasmic reticulum is required for pero xisome biogenesis and suggest that in Y. lipolytica, the trafficking o f some membrane proteins, but not matrix proteins, to the peroxisome o ccurs via the endoplasmic reticulum, results in their glycosylation wi thin the lumen of the endoplasmic reticulum, does not involve transpor t through the Golgi, and requires the products encoded by the SEC238, SRP54, PEX1, and PEX6 genes.