UBE3C與TRABID拮抗調控VPS34的K29/K48分支型泛素化，藉此調控細胞自噬、蛋白質恆定與肝臟代謝

Abstract

泛素-蛋白酶體系統和細胞自噬是兩個主要的細胞品管路徑，它們的功能紊亂皆牽連著各式各樣的疾病。然而，此兩路徑的交互影響仍然未被透徹了解。在本篇研究中，我們發現泛素連接酶UBE3C和去泛素化酶TRABID相互拮抗調控VPS34的K29/K48分支型泛素化，從而平衡地維持基本與營養缺失誘導的細胞自噬活性。我們研究指出此泛素化增強了VPS34與蛋白酶體的結合與它的降解，由此抑制自噬小體的生成與成熟。在內質網和蛋白毒性壓力的情況下，位在吞噬泡的UBE3C蛋白量會下降，同時與蛋白酶體結合的UBE3C的蛋白量會上升，營養缺失的條件下則沒有觀察到此現象。此UBE3C位置與蛋白交互作用的改變，減少UBE3C對VPS34的負向調控，從而增加包括蛋白質聚合體自噬與內質網自噬的細胞自噬活性來促進蛋白質恆定、內質網品管和細胞存活。在肝臟，TRABID促使的VPS34蛋白穩定路徑對脂質代謝是非常重要的，脂肪變性病程中會負向調節此分子機制。綜觀來說，本研究證實VPS34的新穎泛素化型態以及闡明此VPS34修飾對細胞蛋白質恆定和生理上的肝臟代謝有重要功能。

Ubiquitin-proteasome system (UPS) and autophagy are two major quality control processes whose impairment is linked to a wide variety of diseases. However, the interplay between UPS and autophagy remains incompletely understood. Here, we show that ubiquitin ligase UBE3C and deubiquitinating enzyme TRABID reciprocally regulate K29/K48 branched ubiquitination of VPS34 to balance basal and nutrient starvation-induced autophagy. This ubiquitination enhances the binding of VPS34 to proteasome for degradation, thereby suppressing autophagosome formation and maturation. Under ER and proteotoxic stresses, but not nutrient starvation UBE3C recruitment to phagophore is compromised with a concomitant increase of its association with proteasomes. This switch attenuates the action of UBE3C on VPS34, thereby elevating autophagy activities including aggrephagy and ER-phagy to facilitate proteostasis, ER quality control and cell survival. In the liver, TRABID-mediated VPS34 stabilization is critical for lipid metabolism and is downregulated during the pathogenesis of steatosis. This study identifies a previously unappreciated ubiquitination type on VPS34 and elucidates its cellular fate and physiological functions in proteostasis and liver metabolism.