PEX20P OF THE YEAST YARROWIA-LIPOLYTICA IS REQUIRED FOR THE OLIGOMERIZATION OF THIOLASE IN THE CYTOSOL AND FOR ITS TARGETING TO THE PEROXISOME

Pex mutants are defective in peroxisome assembly. In the pex20-1 mutan t strain of the yeast Yarrowia lipolytica, the peroxisomal matrix prot ein thiolase is mislocalized exclusively to the cytosol, whereas the i mport of other peroxisomal proteins is unaffected. The PEX20 gene was isolated by functional complementation of the pex20-1 strain and encod es a protein, Pex20p, of 424 amino acids (47,274 D). Despite its role in the peroxisomal import of thiolase, which is targeted by an amino-t erminal peroxisomal targeting signal-2 (PTS2), Pex20p does not exhibit homology to Pex7p, which acts as the PTS2 receptor. Pex20p is mostly cytosolic, whereas 4-8% is associated with high-speed (200,000 g) pell etable peroxisomes. In the wild-type strain, all newly synthesized thi olase is associated with Pex20p in a heterotetrameric complex composed of two polypeptide chains of each protein. This association is indepe ndent of PTS2. Pex20p is required for both the oligomerization of thio lase in the cytosol and its targeting to the peroxisome. Our data sugg est that monomeric Pex20p binds newly synthesized monomeric thiolase i n the cytosol and promotes the formation of a heterotetrameric complex of these two proteins, which could further bind to the peroxisomal me mbrane. Translocation of the thiolase homodimer into the peroxisomal m atrix would release Pex20p monomers back to the cytosol, thereby permi tting a new cycle of binding-oligomerization-targeting-release for Pex 20p and thiolase.