Reducing PEX13 Expression Ameliorates Physiological Defects of Late-Acting Peroxin Mutants

Department of Biochemistry and Cell Biology, Rice University, Houston, TX 77005, USA.
Traffic (Impact Factor: 4.35). 10/2010; 12(1):121-34. DOI: 10.1111/j.1600-0854.2010.01136.x
Source: PubMed


Proteins are targeted to the peroxisome matrix via processes that are mechanistically distinct from those used by other organelles. Protein entry into peroxisomes requires peroxin (PEX) proteins, including early-acting receptor (e.g. PEX5) and docking peroxins (e.g. PEX13 and PEX14) and late-acting PEX5-recycling peroxins (e.g. PEX4 and PEX6). We examined genetic interactions among Arabidopsis peroxin mutants and found that the weak pex13-1 allele had deleterious effects when combined with pex5-1 and pex14-2, which are defective in early-acting peroxins, as shown by reduced matrix protein import and enhanced physiological defects. In contrast, combining pex13-1 with pex4-1 or pex6-1, which are defective in late-acting peroxins, unexpectedly ameliorated mutant growth defects. Matrix protein import remained impaired in pex4-1 pex13-1 and pex6-1 pex13-1, suggesting that the partial suppression of pex4-1 and pex6-1 physiological defects by a weak pex13 allele may result from restoring the balance between import and export of PEX5 or other proteins that are retrotranslocated from the peroxisome with the assistance of PEX4 and PEX6. Our results suggest that symptoms caused by pex mutants defective in late-acting peroxins may result not only from defects in matrix protein import but also from inefficient removal of PEX5 from the peroxisomal membrane following cargo delivery.

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Available from: Andrew W Woodward, Nov 13, 2015
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    • "This is in line with the conceptual idea that import and export rates of the receptors have to be balanced because the binding capacities of the peroxisomal membrane complexes seem to be restricted [19], [56], [58]. This connection has been demonstrated for A. thaliana, where a strain expressing an functionally impaired PEX6 mutant was only able to facilitate matrix protein import, when this late-acting mutant was combined with an early-acting-mutant, which was a low-expressed allele of the docking protein PEX13 [59]. In line with this, a recently published stochastic computational model of the dynamics of Pex5p ubiquitination and matrix protein translocation in mammals suggests that both processes are cooperatively coupled [60]. "
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    ABSTRACT: Peroxisomal biogenesis is an ubiquitin-dependent process because the receptors required for the import of peroxisomal matrix proteins are controlled via their ubiquitination status. A key step is the monoubiquitination of the import receptor Pex5p by the ubiquitin-conjugating enzyme (E2) Pex4p. This monoubiquitination is supposed to take place after Pex5p has released the cargo into the peroxisomal matrix and primes Pex5p for the extraction from the membrane by the mechano-enzymes Pex1p/Pex6p. These two AAA-type ATPases export Pex5p back to the cytosol for further rounds of matrix protein import. Recently, it has been reported that the soluble Pex4p requires the interaction to its peroxisomal membrane-anchor Pex22p to display full activity. Here we demonstrate that the soluble C-terminal domain of Pex22p harbours its biological activity and that this activity is independent from its function as membrane-anchor of Pex4p. We show that Pex4p can be functionally fused to the trans-membrane segment of the membrane protein Pex3p, which is not directly involved in Pex5p-ubiquitination and matrix protein import. However, this Pex3(N)-Pex4p chimera can only complement the double-deletion strain pex4Δ/pex22Δ and ensure optimal Pex5p-ubiquitination when the C-terminal part of Pex22p is additionally expressed in the cell. Thus, while the membrane-bound portion Pex22(N)p is not required when Pex4p is fused to Pex3(N)p, the soluble Pex22(C)p is essential for peroxisomal biogenesis and efficient monoubiquitination of the import receptor Pex5p by the E3-ligase Pex12p in vivo and in vitro. The results merge into a picture of an ubiquitin-conjugating complex at the peroxisomal membrane consisting of three domains: the ubiquitin-conjugating domain (Pex4p), a membrane-anchor domain (Pex22(N)p) and an enhancing domain (Pex22(C)p), with the membrane-anchor domain being mutually exchangeable, while the Ubc- and enhancer-domains are essential.
    Full-text · Article · Aug 2014 · PLoS ONE
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    • "The attenuation of receptor export by functional impairment of the exportomer results in an accumulation of S. cerevisiae receptors at the membrane (Platta et al., 2004; Kiel et al., 2005) and therefore prevents the docking of incoming receptor-cargo complexes arriving from the cytosol. This assumption is in line with recent data from work in A. thaliana (Ratzel et al., 2011). The physiological defects of mutated and only insufficiently active Pex6p were partially restored when combined with a weakly expressed allele of the docking protein Pex13p (Ratzel et al., 2011). "
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    ABSTRACT: Peroxisomes constitute a dynamic compartment in all nucleated cells. They fulfill diverse metabolic tasks in response to environmental changes and cellular demands. This adaptation is implemented by modulation of the enzyme content of the organelles, which is accomplished by dynamically operating peroxisomal protein transport machineries. Soluble import receptors recognize their newly synthesized cargo proteins in the cytosol and ferry them to the peroxisomal membrane. Subsequently, the cargo is translocated into the matrix, where the receptor is ubiquitinated and exported back to the cytosol for further rounds of matrix protein import. This review discusses the recent progress in our understanding of the peroxisomal matrix protein import and its regulation by ubiquitination events as well as the current view on the translocation mechanism of folded proteins into peroxisomes. This article is part of a Special Issue entitled: Origin and spatiotemporal dynamics of the peroxisomal endomembrane system.
    Full-text · Article · Sep 2013 · Frontiers in Physiology
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    • "In addition to low PEX5 levels observed in pex6-1 mutants (Zolman et al., 2005; Ratzel et al., 2011), PEX5 levels are reduced in light-grown pex7 mutants (Ramón and Bartel, 2010), suggesting that the dependence of PEX7 on PEX5 for cargo delivery in plants (Hayashi et al., 2005; Woodward and Bartel, 2005a) is mirrored by a dependence of PEX5 on PEX7 for stability . Whether the apparent PEX5 instability in pex7 mutants reflects inefficient recycling leading to RADAR or instability in the cytosol remains to be determined. "
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    ABSTRACT: Peroxisomes are eukaryotic organelles that are highly dynamic both in morphology and metabolism. Plant peroxisomes are involved in numerous processes, including primary and secondary metabolism, development, and responses to abiotic and biotic stresses. Considerable progress has been made in the identification of factors involved in peroxisomal biogenesis, revealing mechanisms that are both shared with and diverged from non-plant systems. Furthermore, recent advances have begun to reveal an unexpectedly large plant peroxisomal proteome and have increased our understanding of metabolic pathways in peroxisomes. Coordination of the biosynthesis, import, biochemical activity, and degradation of peroxisomal proteins allows for highly dynamic responses of peroxisomal metabolism to meet the needs of a plant. Knowledge gained from plant peroxisomal research will be instrumental to fully understanding the organelle's dynamic behavior and defining peroxisomal metabolic networks, thus allowing the development of molecular strategies for rational engineering of plant metabolism, biomass production, stress tolerance, and pathogen defense.
    Full-text · Article · Jun 2012 · The Plant Cell
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