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J. Cell Biol.,
Volume 140, Number 4, February 23, 1998 807-820
* Division of Cellular and Molecular Medicine and Abstract. Using a new screening procedure for the isolation of peroxisomal import mutants in Pichia pastoris,
we have isolated a mutant (pex7) that is specifically disturbed in the peroxisomal import of proteins containing
a peroxisomal targeting signal type II (PTS2). Like its
Saccharomyces cerevisiae homologue, PpPex7p interacted with the PTS2 in the two-hybrid system, suggesting that Pex7p functions as a receptor. The pex7 Department of Biology, University of California at San Diego, La Jolla,
California 92093-0322
mutant was not impaired for growth on methanol,
indicating that there are no PTS2-containing enzymes
involved in peroxisomal methanol metabolism. In contrast, pex7 cells failed to grow on oleate, but growth
on oleate could be partially restored by expressing thiolase (a PTS2-containing enzyme) fused to the PTS1.
Because the subcellular location and mechanism of action of this protein are controversial, we used various
methods to demonstrate that Pex7p is both cytosolic
and intraperoxisomal. This suggests that Pex7p functions as a mobile receptor, shuttling PTS2-containing
proteins from the cytosol to the peroxisomes. In addition, we used PpPex7p as a model protein to understand the effect of the Pex7p mutations found in human patients with rhizomelic chondrodysplasia punctata.
The corresponding PpPex7p mutant proteins were stably expressed in P. pastoris, but they failed to complement the pex7 mutant and were impaired in binding
to the PTS2 sequence.
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