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Deciphering the machinery controlling the degradation of oleosins, the gatekeepers of seed lipid droplets
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International audience. Seed lipid droplets (LDs) of many dicotyledonous plants, including Arabidopsis thaliana, are a good model to investigate the mechanisms controlling LD dynamics. Indeed, LD degradation occurs rapidly and massively in germinated seeds, to provide carbon and energy for seedling growth. Post-germinative mobilization of storage lipids is preceded by the degradation of oleosins, the major structural LD proteins which stabilize LDs in dry seeds. We previously showed that oleosins are marked for degradation by ubiquitination, and extracted from LDs before proteolysis. The mechanisms underlying the dislocation of these LD-anchored proteins from the phospholipid monolayer are yet unknown. We found that a protein called PUX10 is required for proper extraction of ubiquitinated oleosins from the LD surface in young Arabidopsis seedlings. PUX10 localizes to LDs and binds to ubiquitinated proteins. PUX10 also interacts with CDC48A, the ATPase which selectively extract misfolded proteins from the ER in the degradation process known as ERAD (for ER-associated degradation). We propose that PUX10 is an adaptor protein recruiting CDC48A to ubiquitinated oleosins, thus promoting the extraction of oleosins from LDs by the segregase activity of CDC48A. PUX10 and CDC48A are core components of a new LD-associated machinery that we named LD-associated degradation (LDAD) system. Moreover, we observed that PUX10 and CDC48A localize to a specific subpopulation of LDs in germinated seeds, suggesting a functional differentiation of LDs in plants. We also showed that a deubiquitinating enzyme known for its role in intracellular trafficking and vacuolar biogenesis is necessary for the ubiquitination of oleosins.
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