The Early-Acting Peroxin PEX19 Is Redundantly Encoded, Farnesylated, and Essential for Viability in Arabidopsis thaliana

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.过氧化物酶体（Peroxisomes）是单层膜包被的细胞器，对动植物的正常发育至关重要。在哺乳动物和酵母中，过氧化物酶体蛋白（peroxin, PEX）PEX3与PEX19介导过氧化物酶体膜蛋白（peroxisome membrane protein, PMP）插入以及前过氧化物酶体从内质网出芽的早期步骤。PEX3膜蛋白可作为PEX19的停靠位点，PEX19是一类针对PMP的胞质分子伴侣，负责将PMP递送至内质网或过氧化物酶体膜。在酵母和哺乳动物中，PEX19会发生法尼基化修饰；我们借助结合异戊二烯化突变体的免疫印迹实验证实，野生型拟南芥（Arabidopsis thaliana）中的PEX19同样完全发生法尼基化修饰。我们针对拟南芥的两种PEX19同工型PEX19A与PEX19B分别构建了插入突变等位基因，在pex19a-1突变体中检测到的PEX19蛋白水平与野生型相近，但pex19b-1突变体中几乎无法检测到PEX19蛋白。尽管pex19b-1突变体的PEX19水平有所降低，但pex19a-1与pex19b-1单突变体均未表现出显著的过氧化物酶体β-氧化缺陷，且过氧化物酶体基质蛋白与膜蛋白的水平及定位均正常。pex19a-1 pex19b-1双突变体表现为胚胎致死，说明PEX19在胚胎发生过程中存在功能冗余的关键作用。表达任一PEX19同工型的YFP标记版本均可挽救该致死表型，证实PEX19A与PEX19B在拟南芥中功能冗余。我们观察到，pex19b-1可加剧部分过氧化物酶体蛋白缺陷突变体的过氧化物酶体相关缺陷，这支持PEX19在过氧化物酶体功能中发挥作用的结论。综上，本研究数据表明，拟南芥PEX19可促进过氧化物酶体功能，且对植株存活至关重要。