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The ubiquitin-conjugating enzyme Pex4p of Hansenula polymorpha is required for efficient functioning of the PTS1
I J van der Klei1, R E Hilbrands, J A Kiel
1Eukaryotic Microbiology, Groningen Biomolecular Sciences and Biotechnology Institute, University of Groningen, Biological Centre, Kerklaan 30, 9751 NN Haren, The Netherlands. ijvdklei@biol.rug.nl
Insights
The Hansenula polymorpha PEX4 gene product, Pex4p, is crucial for peroxisomal matrix protein import. Pex4p likely facilitates the recycling of the PTS1 receptor, Pex5p, from peroxisomes back to the cytosol.
Area of Science:
- Cell Biology
- Molecular Biology
- Biochemistry
Background:
- Peroxisomes are vital organelles involved in various metabolic processes.
- Protein import into peroxisomes is a complex process involving specific targeting signals and receptors.
- The ubiquitin-conjugating enzyme family plays diverse roles in cellular regulation.
Purpose of the Study:
- To clone and characterize the Hansenula polymorpha PEX4 gene.
- To investigate the role of Pex4p in peroxisomal protein import.
- To elucidate the mechanism of Pex5p recycling in H. polymorpha.
Main Methods:
- Functional complementation of a peroxisome-deficient mutant.
- Gene deletion and strain construction (Deltapex4).
- Analysis of peroxisomal protein import using PTS1 and PTS2 signals.
- Localization studies of Pex5p in wild-type and mutant cells.
Main Results:
- Cloning of the H. polymorpha PEX4 gene, encoding a ubiquitin-conjugating enzyme Pex4p.
- Deltapex4 mutant exhibits a specific defect in the import of peroxisomal matrix proteins with a PTS1 signal.
- Overproduction of Pex5p suppresses the PTS1 import defect, with Pex5p accumulating at the peroxisomal membrane.
Conclusions:
- Pex4p is essential for the functional recycling of the PTS1 receptor, Pex5p, from peroxisomes to the cytosol.
- Pex4p's role is likely involved in mediating the dissociation or release of Pex5p from the peroxisomal membrane.
- This study provides insights into the intricate mechanisms governing peroxisomal protein import and receptor trafficking.
Abstract:
We have cloned the Hansenula polymorpha PEX4 gene by functional complementation of a peroxisome-deficient mutant. The PEX4 translation product, Pex4p, is a member of the ubiquitin-conjugating enzyme family. In H.polymorpha, Pex4p is a constitutive, low abundance protein. Both the original mutant and the pex4 deletion strain (Deltapex4) showed a specific defect in import of peroxisomal matrix proteins containing a C-terminal targeting signal (PTS1) and of malate synthase, whose targeting signal is not yet known. Import of the PTS2 protein amine oxidase and the insertion of the peroxisomal membrane proteins Pex3p and Pex14p was not disturbed in Deltapex4 cells. The PTS1 protein import defect in Deltapex4 cells could be suppressed by overproduction of the PTS1 receptor, Pex5p, in a dose-response related manner. In such cells, Pex5p is localized in the cytosol and in peroxisomes. The peroxisome-bound Pex5p specifically accumulated at the inner surface of the peroxisomal membrane and thus differed from Pex5p in wild-type peroxisomes, which is localized throughout the matrix. We hypothesize that in H. polymorpha Pex4p plays an essential role for normal functioning of Pex5p, possibly in mediating recycling of Pex5p from the peroxisome to the cytosol.