4 DISTINCT SECRETORY PATHWAYS SERVE PROTEIN SECRETION, CELL-SURFACE GROWTH, AND PEROXISOME BIOGENESIS IN THE YEAST YARROWIA-LIPOLYTICA

We have identified and characterized mutants of the yeast Yarrowia lip olytica that are deficient in protein secretion, in the ability to und ergo dimorphic transition from the yeast to the mycelial form, and in peroxisome biogenesis. Mutations in the SEC238, SRP54, PEX1, PEX2, PEX 6, and PEX9 genes affect protein secretion, prevent the exit of the pr ecursor form of alkaline extracellular protease from the endoplasmic r eticulum, and compromise peroxisome biogenesis. The mutants sec238A, s rp54KO, pex2KO, pex6KO, and pex9KO are also deficient in the dimorphic transition front the yeast to the mycelial form and are affected in t he export of only plasma membrane and cell wail-associated proteins sp ecific for the mycelial form, Mutations in the SEC238, SRP54, PEX2, an d PEX6 genes prevent or significantly delay the exit of two peroxisoma l membrane proteins, Pex2p and Pex16p, from the endoplasmic reticulum en mute to the peroxisomal membrane. Mutations in the PEX5, PEX16, and PEX17 genes, which have previously been shown to be essential for per oxisome 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 p roteins 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 cel l nail synthesis during yeast and mycelial modes of growth, and peroxi some biogenesis. At least two of these secretory pathways, which are i nvolved in tile export of proteins to the external medium and in the d elivery of proteins for assembly of the peroxisomal membrane, diverge at the level of the endoplasmic reticulum.