The ubiquitin-protein ligase MIEL1 localizes to peroxisomes to promote seedling oleosin degradation and lipid droplet mobilization.

Abstract

Lipid droplets are organelles conserved across eukaryotes that store and release neutral lipids to regulate energy homeostasis. In oilseed plants, fats stored in seed lipid droplets provide fixed carbon for seedling growth before photosynthesis begins. As fatty acids released from lipid droplet triacylglycerol are catabolized in peroxisomes, lipid droplet coat proteins are ubiquitinated, extracted, and degraded. In <i>Arabidopsis</i> seeds, the predominant lipid droplet coat protein is OLEOSIN1 (OLE1). To identify genes modulating lipid droplet dynamics, we mutagenized a line expressing mNeonGreen-tagged OLE1 expressed from the OLE1 promoter and isolated mutants with delayed oleosin degradation. From this screen, we identified four <i>miel1</i> mutant alleles. MIEL1 (MYB30-interacting E3 ligase 1) targets specific MYB transcription factors for degradation during hormone and pathogen responses [D. Marino <i>et al</i>., <i>Nat. Commun.</i> <b>4</b>, 1476 (2013); H. G. Lee and P. J. Seo, <i>Nat. Commun.</i> <b>7</b>, 12525 (2016)] but had not been implicated in lipid droplet dynamics. <i>OLE1</i> transcript levels were unchanged in <i>miel1</i> mutants, indicating that MIEL1 modulates oleosin levels posttranscriptionally. When overexpressed, fluorescently tagged MIEL1 reduced oleosin levels, causing very large lipid droplets. Unexpectedly, fluorescently tagged MIEL1 localized to peroxisomes. Our data suggest that MIEL1 ubiquitinates peroxisome-proximal seed oleosins, targeting them for degradation during seedling lipid mobilization. The human MIEL1 homolog (PIRH2; p53-induced protein with a RING-H2 domain) targets p53 and other proteins for degradation and promotes tumorigenesis [A. Daks <i>et al</i>., <i>Cells</i> <b>11</b>, 1515 (2022)]. When expressed in <i>Arabidopsis</i>, human PIRH2 also localized to peroxisomes, hinting at a previously unexplored role for PIRH2 in lipid catabolism and peroxisome biology in mammals.