Abstract
Peroxisomes are single-membrane bound organelles that are essential for normal development in plants and animals. In mammals and yeast, the peroxin (PEX) proteins PEX3 and PEX19 facilitate the early steps of peroxisome membrane protein (PMP) insertion and pre-peroxisome budding from the endoplasmic reticulum. The PEX3 membrane protein acts as a docking site for PEX19, a cytosolic chaperone for PMPs that delivers PMPs to the endoplasmic reticulum or peroxisomal membrane. PEX19 is farnesylated in yeast and mammals, and we used immunoblotting with prenylation mutants to show that PEX19 also is fully farnesylated in wild-type Arabidopsis thaliana plants. We examined insertional alleles disrupting either of the two Arabidopsis PEX19 isoforms, PEX19A or PEX19B, and detected similar levels of PEX19 protein in the pex19a-1 mutant and wild type; however, PEX19 protein was nearly undetectable in the pex19b-1 mutant. Despite the reduction in PEX19 levels in pex19b-1, both pex19a-1 and pex19b-1 single mutants lacked notable peroxisomal β-oxidation defects and displayed normal levels and localization of peroxisomal matrix and membrane proteins. The pex19a-1 pex19b-1 double mutant was embryo lethal, indicating a redundantly encoded critical role for PEX19 during embryogenesis. Expressing YFP-tagged versions of either PEX19 isoform rescued this lethality, confirming that PEX19A and PEX19B act redundantly in Arabidopsis. We observed that pex19b-1 enhanced peroxisome-related defects of a subset of peroxin-defective mutants, supporting a role for PEX19 in peroxisome function. Together, our data indicate that Arabidopsis PEX19 promotes peroxisome function and is essential for viability.
Highlights
Peroxisomes are eukaryotic organelles that house critical oxidative reactions and sequester harmful reactive oxygen species to prevent damage to other cellular compartments
We found that the levels of these peroxisome membrane protein (PMP) were similar to wild type in pex19a-1, pex19b-1, the prenylation mutants, and the 35:HA-PEX19 lines (Fig 4C), suggesting that altering PEX19 levels or prenylation did not dramatically alter PMP stability in Arabidopsis
We found PEX14 and APX3 in the pellet fraction in wild type, pex19a-1, and pex19b-1 (Fig 5), suggesting that these PMPs remain membrane-associated despite the low PEX19 protein levels in the pex19b-1 mutant
Summary
Peroxisomes are eukaryotic organelles that house critical oxidative reactions and sequester harmful reactive oxygen species to prevent damage to other cellular compartments. Peroxisomal enzymes participate in diverse metabolic processes, including photorespiration and fatty acid β-oxidization (reviewed in [1]). Peroxin (PEX) proteins function in de novo peroxisome biogenesis, division, and matrix protein import. Peroxisome matrix proteins are synthesized in the cytosol and usually are targeted to the peroxisome by a carboxyl-terminal three-amino acid peroxisome-targeting signal 1 (PTS1), which is recognized by the cytosolic PTS1 receptor, PEX5 [12]. A less common targeting mechanism uses an N-terminal nine-amino acid PTS2, which is recognized by the cytosolic PTS2 receptor, PEX7 [13, 14]. PEX5 forms part of a transient pore that facilitates cargo entry into the peroxisome [16], after which PEX5 is recycled with the assistance of PEX4, a ubiquitin-conjugating enzyme, and the ubiquitin-protein ligases PEX2, PEX10, and PEX12 (reviewed in [15]). Retrotranslocated PEX5 can be poly-ubiquitinated and degraded by the proteasome [15]
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