Abstract
BackgroundPeroxisomes house critical metabolic reactions. For example, fatty acid β-oxidation enzymes, which are essential during early seedling development, are peroxisomal. Peroxins (PEX proteins) are needed to bring proteins into peroxisomes. Most matrix proteins are delivered to peroxisomes by PEX5, a receptor that forms transient pores to escort proteins across the peroxisomal membrane. After cargo delivery, a peroxisome-tethered ubiquitin-conjugating enzyme (PEX4) and peroxisomal ubiquitin-protein ligases mono- or polyubiquitinate PEX5 for recycling back to the cytosol or for degradation, respectively. Arabidopsis pex mutants β-oxidize fatty acids inefficiently and therefore fail to germinate or grow less vigorously. These defects can be partially alleviated by providing a fixed carbon source, such as sucrose, in the growth medium. Despite extensive characterization of peroxisome biogenesis in Arabidopsis grown in non-challenged conditions, the effects of environmental stressors on peroxisome function and pex mutant dysfunction are largely unexplored.ResultsWe surveyed the impact of growth temperature on a panel of pex mutants and found that elevated temperature ameliorated dependence on external sucrose and reduced PEX5 levels in the pex4-1 mutant. Conversely, growth at low temperature exacerbated pex4-1 physiological defects and increased PEX5 levels. Overexpressing PEX5 also worsened pex4-1 defects, implying that PEX5 lingering on the peroxisomal membrane when recycling is impaired impedes peroxisome function. Growth at elevated temperature did not reduce the fraction of membrane-associated PEX5 in pex4-1, suggesting that elevated temperature did not restore PEX4 enzymatic function in the mutant. Moreover, preventing autophagy in pex4-1 did not restore PEX5 levels at high temperature. In contrast, MG132 treatment increased PEX5 levels, implicating the proteasome in degrading PEX5, especially at high temperature.ConclusionsWe conclude that growth at elevated temperature increases proteasomal degradation of PEX5 to reduce overall PEX5 levels and ameliorate pex4-1 physiological defects. Our results support the hypothesis that efficient retrotranslocation of PEX5 after cargo delivery is needed not only to make PEX5 available for further rounds of cargo delivery, but also to prevent the peroxisome dysfunction that results from PEX5 lingering in the peroxisomal membrane.Electronic supplementary materialThe online version of this article (doi:10.1186/s12870-015-0605-3) contains supplementary material, which is available to authorized users.
Highlights
IntroductionMost matrix proteins are delivered to peroxisomes by PEX5, a receptor that forms transient pores to escort proteins across the peroxisomal membrane
This restoration of sucrose independence by growth at high temperature was specific to pex4-1; the sucrose dependence of pex5-1, pex7-2, pex14-1, pex2-1 and pex10-2 was unchanged or very slightly exacerbated at high temperature, and pex13-4 did not germinate without sucrose at either temperature (Fig. 1a)
We focused on the pex4-1 mutant to elucidate the molecular changes in peroxisome function that accompany growth at high temperature
Summary
Most matrix proteins are delivered to peroxisomes by PEX5, a receptor that forms transient pores to escort proteins across the peroxisomal membrane. Arabidopsis pex mutants β-oxidize fatty acids inefficiently and fail to germinate or grow less vigorously These defects can be partially alleviated by providing a fixed carbon source, such as sucrose, in the growth medium. Like Arabidopsis thaliana, β-oxidize fatty acids to provide energy for early seedling development before photosynthesis is established [1]. Because this β-oxidation is peroxisomal, dependence on an external source of fixed carbon, such as sucrose, during germination is a hallmark of peroxisome-defective mutants [2, 3]. The receptor-cargo complexes translocate cargo with the assistance of docking peroxins (PEX13 and PEX14); this importomer forms transient pores on the peroxisomal membrane to deliver cargo into the peroxisome matrix [10, 11]
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