Yarrowia lipolytica cells mutant for the peroxisomal peroxin Pex19p contain structures resembling wild-type peroxisomes

PEX genes encode peroxins, which are proteins required for peroxisome assem bly. The PEX19 gene of the yeast Yarrowia lipolytica was isolated by functi onal complementation of the oleic acid-nonutilizing strain pex19-1 and enco des Pex19p, a protein of 324 amino acids (34,822 Da). Subcellular fractiona tion and immunofluorescence microscopy showed Pex19p to be localized primar ily to peroxisomes. Pex19p is detected in cells grown mi glucose-containing medium, and its levels are not increased by incubation of cells in oleic a cid-containing medium, the metabolism of which requires intact peroxisomes. pex19 cells preferentially mislocalize peroxisomal matrix proteins and the peripheral intraperoxisomal membrane peroxin Pex16p to the cytosol, althou gh small amounts of these proteins could be reproducibly localized to a sub cellular fraction enriched for peroxisomes. In contrast, the peroxisomal in tegral membrane protein Pex2p exhibits greatly reduced levels in pex19 cell s compared with its levels in wild-type cells. Importantly, pex19 cells wer e shown by electron microscopy to contain structures that resemble wild-typ e peroxisomes in regards to size, shape, number, and electron density. Subc ellular fractionation and isopycnic density gradient centrifugation confirm ed the presence of vesicular structures in pex19 mutant strains that were s imilar in density to wild-type peroxisomes and that contained profiles of p eroxisomal matrix and membrane proteins that are similar to, yet distinct f rom, those of wild-type peroxisomes. Because peroxisomal structures form in pex19 cells, Pex19p apparently does not function as a peroxisomal membrane protein receptor in Y. lipolytica. Our results are consistent with a role for Y. lipolytica Pex19p in stabilizing the peroxisomal membrane.