A <i>pex1</i> missense mutation improves peroxisome function in a subset of <i>Arabidopsis pex6</i> mutants without restoring PEX5 recycling

Abstract

Peroxisomes are eukaryotic organelles critical for plant and human development because they house essential metabolic functions, such as fatty acid β-oxidation. The interacting ATPases PEX1 and PEX6 contribute to peroxisome function by recycling PEX5, a cytosolic receptor needed to import proteins targeted to the peroxisomal matrix. <i>Arabidopsis pex6</i> mutants exhibit low PEX5 levels and defects in peroxisomal matrix protein import, oil body utilization, peroxisomal metabolism, and seedling growth. These defects are hypothesized to stem from impaired PEX5 retrotranslocation leading to PEX5 polyubiquitination and consequent degradation of PEX5 via the proteasome or of the entire organelle via autophagy. We recovered a <i>pex1</i> missense mutation in a screen for second-site suppressors that restore growth to the <i>pex6-1</i> mutant. Surprisingly, this <i>pex1-1</i> mutation ameliorated the metabolic and physiological defects of <i>pex6-1</i> without restoring PEX5 levels. Similarly, preventing autophagy by introducing an <i>atg7</i>-null allele partially rescued <i>pex6-1</i> physiological defects without restoring PEX5 levels. <i>atg7</i> synergistically improved matrix protein import in <i>pex1-1 pex6-1</i>, implying that <i>pex1-1</i> improves peroxisome function in <i>pex6-1</i> without impeding autophagy of peroxisomes (i.e., pexophagy). <i>pex1-1</i> differentially improved peroxisome function in various <i>pex6</i> alleles but worsened the physiological and molecular defects of a <i>pex26</i> mutant, which is defective in the tether anchoring the PEX1-PEX6 hexamer to the peroxisome. Our results support the hypothesis that, beyond PEX5 recycling, PEX1 and PEX6 have additional functions in peroxisome homeostasis and perhaps in oil body utilization.