Pex22p of Pichia pastoris, essential for peroxisomal matrix protein import, anchors the ubiquitin-conjugating enzyme, Pex4p, on the peroxisomal membrane

We isolated a Pichia pastoris mutant: that was unable to grow on the peroxi some-requiring media, methanol and oleate. Cloning the gene by complementat ion revealed that the encoded protein, Pex22p, is a new peroxin. A Delta pe x22 strain does not grow on methanol or oleate and is unable to import pero xisomal matrix proteins. However, this strain targets peroxisomal membrane proteins to membranes, most likely peroxisomal remnants, detectable by fluo rescence and electron microscopy. Pex22p, composed of 187 amino acids, is a n integral peroxisomal membrane protein with its NH2 terminus in the matrix and its COOH terminus in the cytosol. It contains a 25-amino acid peroxiso me membrane-targeting signal at its NH2 terminus. Pex22p interacts with the ubiquitin-conjugating enzyme Pex4p, a peripheral peroxisomal membrane prot ein, in vivo, and in a yeast two-hybrid experiment. Pex22p is required for the peroxisomal localization of Pex4p and in strains lacking Pex22p, the Pe x4p is cytosolic and unstable. Therefore, Pex22p anchors Pex4p at the perox isomal membrane. Strains that do not express Pex4p or Pex22p have similar p henotypes and lack Pex5p, suggesting that Pex4p and Pex22p act at the same step in peroxisome biogenesis. The Saccharomyces cerevisiae hypothetical pr otein, Yaf5p, is the functional homologue of P. pastoris Pex22p.