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 Arabidopsis 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 miel1 mutant alleles. MIEL1 (MYB30-interacting E3 ligase 1) targets specific MYB transcription factors for degradation during hormone and pathogen responses [D. Marino et al ., Nat. Commun. 4 , 1476 (2013); H. G. Lee and P. J. Seo, Nat. Commun. 7 , 12525 (2016)] but had not been implicated in lipid droplet dynamics. OLE1 transcript levels were unchanged in miel1 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 et al ., Cells 11 , 1515 (2022)]. When expressed in Arabidopsis , human PIRH2 also localized to peroxisomes, hinting at a previously unexplored role for PIRH2 in lipid catabolism and peroxisome biology in mammals.

Degeneration of oil bodies by rough endoplasmic reticulum (rER)-associated protein during seed germination in Cannabis sativa L.

“…Recently, some researchers reported on the LD degradation system in plants ( Farquharson 2018 ; Kretzschmar et al 2018 ; Traver and Bartel 2023 ). Since LDs are strongly associated with the endoplasmic reticulum (ER) ( Suzuki 2017 ), this association has been observed at the electron microscopy level in many organisms ( Fujimoto et al 2013 ; Ohsaki et al 2017 ; Sui et al 2018 ; Renne et al 2020 ; Kang et al 2022 ; Zhang et al 2022 ).…”

Profile of Bonnie Bartel

“…For Bartel’s IA ( 1 ), she and graduate student Melissa Traver discovered that a ubiquitin-protein ligase, MIEL1, which was known to mark nuclear transcription factors for degradation, also plays a role in the partnership between peroxisomes and lipid droplets. Lipid droplets are coated with proteins, including oleosin, to prevent the droplets from coalescing.…”

“…The work has elucidated regulation and signaling of the plant growth hormone auxin, revealed some of the first known plant microRNAs, and advanced understanding of peroxisome biology. Her Inaugural Article ( 1 ) illuminates the interactions between peroxisomes, which catabolize fats, and lipid droplets, which store fats. Because this interorganelle cooperation occurs in nearly all eukaryotic cells, the findings provide insights into human metabolic diseases, agriculture, and biotechnology.…”

Molecular Machinery of Lipid Droplet Degradation and Turnover in Plants

“…These proteins could be ubiquitinated at the C-terminal lysines [19]. Recently, using forward-genetic screening, MYB30-interacting E3 ligase 1 (MIEL1) was confirmed to ubiquitinate oleosin for degradation in Arabidopsis thaliana [48]. MIEL1 is a ubiquitinprotein ligase known to ubiquitinate two MYB transcription factors, MYB30 and MYB96, for degradation during hormone and pathogen responses [49,50].…”

“…Surprisingly, MIEL1 localizes to peroxisomes, and miel1 mutants delay LD mobilization without increasing oleosin transcript levels, suggesting MIEL1 regulates oleosin post-transcriptionally. Transgenic experiments and protein immunoblot suggest that MIEL1 directly ubiquitinates seed oleosins in the vicinity of a peroxisome for subsequent proteasomal degradation during seedling lipid mobilization [48].…”