Four Distinct Secretory Pathways Serve Protein Secretion, Cell Surface Growth, and Peroxisome Biogenesis in the Yeast Yarrowia lipolytica

Abstract

We have identified and characterized mutants of the yeast Yarrowia lipolytica that are deficient in protein secretion, in the ability to undergo dimorphic transition from the yeast to the mycelial form, and in peroxisome biogenesis. Mutations in the SEC238, SRP54, PEX1, PEX2, PEX6, and PEX9 genes affect protein secretion, prevent the exit of the precursor form of alkaline extracellular protease from the endoplasmic reticulum, and compromise peroxisome biogenesis. The mutants sec238A, srp54KO, pex2KO, pex6KO, and pex9KO are also deficient in the dimorphic transition from the yeast to the mycelial form and are affected in the export of only plasma membrane and cell wall-associated proteins specific for the mycelial form. Mutations in the SEC238, SRP54, PEX1, and PEX6 genes prevent or significantly delay the exit of two peroxisomal membrane proteins, Pex2p and Pex16p, from the endoplasmic reticulum en route to the peroxisomal membrane. Mutations in the PEX5, PEX16, and PEX17 genes, which have previously been shown to be essential for peroxisome biogenesis, affect the export of plasma membrane and cell wall-associated proteins specific for the mycelial form but do not impair exit from the endoplasmic reticulum of either Pex2p and Pex16p or of proteins destined for secretion. Biochemical analyses of these mutants provide evidence for the existence of four distinct secretory pathways that serve to deliver proteins for secretion, plasma membrane and cell wall synthesis during yeast and mycelial modes of growth, and peroxisome biogenesis. At least two of these secretory pathways, which are involved in the export of proteins to the external medium and in the delivery of proteins for assembly of the peroxisomal membrane, diverge at the level of the endoplasmic reticulum.The secretory pathway of eukaryotic cells consists of a series of morphologically and biochemically distinct membranebound compartments. The classical secretory pathway starts with protein translocation into the lumen of the endoplasmic reticulum (ER). From the ER, secretory proteins are transported within a series of vesicles to and through the Golgi complex and are then either delivered to the cell surface or routed to the endosomal-lysosomal (vacuolar) branch of the pathway (32,43,53). At all stages of the pathway, intercompartmental transport is initiated by the formation of coated vesicles on the donor compartment, followed by uncoating of vesicle intermediates prior to their fusion with a specific acceptor compartment (47,(52)(53)(54). While initial studies suggested the existence of one major pathway of vesicle-mediated protein export to the cell surface through a series of membrane-bound compartments, exceptions to this classical scheme of protein secretion have now been described.First, not all proteins of either mammalian or yeast cells are delivered to the cell surface via the vesicle-mediated secretory pathway. At least two nonclassical secretory pathways have been demonstrated in the yeast Saccharomyces cerevisiae (for a detailed discussion, s...

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