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J. Cell Biol.,
Volume 142, Number 2, July 27, 1998 403-420
Department of Cell Biology and Anatomy, University of Alberta, Edmonton, Alberta T6G 2H7, Canada
Pex mutants are defective in peroxisome assembly. In the pex20-1 mutant strain of the yeast Yarrowia lipolytica, the peroxisomal matrix protein thiolase is mislocalized exclusively to the cytosol, whereas
the import of other peroxisomal proteins is unaffected. The PEX20 gene was isolated by functional complementation of the pex20-1 strain and encodes 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-terminal 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) pelletable peroxisomes. In the wild-type
strain, all newly synthesized thiolase is associated with
Pex20p in a heterotetrameric complex composed of two
polypeptide chains of each protein. This association is
independent of PTS2. Pex20p is required for both the
oligomerization of thiolase in the cytosol and its targeting to the peroxisome. Our data suggest that monomeric Pex20p binds newly synthesized monomeric thiolase in the cytosol and promotes the formation of a
heterotetrameric complex of these two proteins, which
could further bind to the peroxisomal membrane.
Translocation of the thiolase homodimer into the peroxisomal matrix would release Pex20p monomers back to the cytosol, thereby permitting a new cycle of binding-oligomerization-targeting-release for Pex20p and
thiolase.
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