Disparate peroxisome‐related defects in Arabidopsis pex6 and pex26 mutants link peroxisomal retrotranslocation and oil body utilization

Abstract

SUMMARY Catabolism of fatty acids stored in oil bodies is essential for Arabidopsis seed germination and seedling development. This fatty acid breakdown occurs in peroxisomes, organelles that sequester oxidative reactions. Import of peroxisomal enzymes is facilitated by peroxins including PEX5, a receptor that delivers cargo proteins from the cytosol to the peroxisomal matrix. After cargo delivery, a complex of the PEX1 and PEX6 ATPases and the PEX26 tail-anchored membrane protein removes ubiquitinated PEX5 from the peroxisomal membrane. We identified Arabidopsis pex6 and pex26 mutants by screening for inefficient seedling β-oxidation phenotypes. The mutants displayed distinct defects in growth, response to a peroxisomally metabolized auxin precursor, and peroxisomal protein import. The low PEX5 levels in these mutants were increased by proteasome inhibitor treatment or by combining pex26 with peroxisome-associated ubiquitination machinery mutants, suggesting that ubiquitinated PEX5 is degraded by the proteasome when PEX6 or PEX26 function is reduced. Combining pex26 with mutations that increase PEX5 levels either worsened or improved pex26 physiological and molecular defects, depending on the introduced lesion. Moreover, elevating PEX5 levels via a 35S:PEX5 transgene exacerbated pex26 defects and ameliorated the defects of only a subset of pex6 alleles, implying that decreased PEX5 is not the sole molecular deficiency in these mutants. We found peroxisomes clustered around persisting oil bodies in pex6 and pex26 seedlings, suggesting a role for peroxisomal retrotranslocation machinery in oil body utilization. The disparate phenotypes of these pex alleles may reflect unanticipated functions of the peroxisomal ATPase complex.

A PEX5 missense allele preferentially disrupts PTS1 cargo import into Arabidopsis peroxisomes

“…In addition to pex5‐2 , other Arabidopsis mutants with low PEX5 levels include pex6 and pex26 mutants (Gonzalez et al., ; Zolman & Bartel, ), which show defects in retrotranslocating PEX5 from the peroxisomal membrane (Gonzalez et al., ; Ratzel et al., ), and pex7 mutants (Ramón & Bartel, ), which are defective in the PEX5‐interacting PTS2 receptor and show import defects of not only PTS2 proteins but also PTS1 proteins (Ramón & Bartel, ; Woodward & Bartel, ). Interestingly, reducing function of PEX2, one of the peroxisomal RING ubiquitin‐protein ligases, restores PEX5 levels in pex6‐1 and pex26‐1 but not pex7‐1 mutants (Burkhart et al., ; Gonzalez et al., ), suggesting multiple avenues for PEX5 degradation. It will be interesting to learn whether the apparent pex5‐2 instability that we observe can be attributed to the peroxisomal ubiquitination machinery.…”

Peroxisome Function, Biogenesis, and Dynamics in Plants

“…3). The atypical pex6-2 mutant displays elevated PEX5 levels and delayed matrix protein degradation but resembles wild type in most measures of peroxisome function Gonzalez et al, 2017).…”

“…PEX1 and PEX6 are members of the ATPases associated with diverse cellular activities family and function in yeast as a trimer of PEX1-PEX6 dimers (Blok et al, 2015;Ciniawsky et al, 2015;Gardner et al, 2015). The PEX1-PEX6 heterohexamer is tethered to the peroxisome by a tail-anchored protein known as PEX15 in yeast (Elgersma et al, 1997), PEX26 in mammals (Matsumoto et al, 2003), and APEM9/DAYU/PEX26 in plants (Goto et al, 2011;Li et al, 2014;Gonzalez et al, 2017). PEX26 binds PEX1-PEX6 via PEX6 (Birschmann et al, 2003;Matsumoto et al, 2003;Goto et al, 2011).…”

“…pex6-1, pex6-3, and pex6-4 alter residues near the second ATPase domain (Zolman and Bartel, 2004;Gonzalez et al, 2017) and display decreased b-oxidation, delayed oil body utilization, impaired matrix protein import, low PEX5 levels (Zolman and Bartel, 2004;Gonzalez et al, 2017), and increased PEX5 membrane association Gonzalez et al, 2017), implying that PEX5 is degraded when recycling is impaired (Fig. 3).…”

“…The peroxins that mediate PEX5 ubiquitination and retrotranslocation (Fig. 3) resemble enzymes acting in ER-associated degradation (for review, see Schliebs et al, 2010), and evidence is mounting that these receptor-recycling peroxins may ubiquitinate and remove additional clients from the peroxisomal membrane Kao et al, 2016;Gonzalez et al, 2017). Mammalian PEX3 and PEX19 function not only in PMP insertion but also in inserting the lipid droplet-and ER-associated hairpin protein, UBXD8 (Schrul and Kopito, 2016).…”