Ubiquitination and proteasomal degradation of ATG12 regulates its proapoptotic activity

During macroautophagy, conjugation of ATG12 to ATG5 is essential for LC3 lipidation and autophagosome formation. Additionally, ATG12 has ATG5-independent functions in diverse processes including mitochondrial fusion and mitochondrial-dependent apoptosis. In this study, we investigated the regulation of free ATG12. In stark contrast to the stable ATG12–ATG5 conjugate, we find that free ATG12 is highly unstable and rapidly degraded in a proteasome-dependent manner. Surprisingly, ATG12, itself a ubiquitin-like protein, is directly ubiquitinated and this promotes its proteasomal degradation. As a functional consequence of its turnover, accumulation of free ATG12 contributes to proteasome inhibitor-mediated apoptosis, a finding that may be clinically important given the use of proteasome inhibitors as anticancer agents. Collectively, our results reveal a novel interconnection between autophagy, proteasome activity, and cell death mediated by the ubiquitin-like properties of ATG12.

在巨自噬（macroautophagy）过程中，ATG12与ATG5的缀合对于微管相关蛋白1轻链3（LC3）的脂化以及自噬体（autophagosome）的形成至关重要。此外，ATG12还具备不依赖于ATG5的功能，参与包括线粒体融合、线粒体依赖性细胞凋亡在内的多种生物学进程。本研究围绕游离态ATG12的调控机制展开探究。与稳定存在的ATG12–ATG5缀合物形成鲜明对比的是，我们发现游离态ATG12极不稳定，会以蛋白酶体依赖性方式被快速降解。令人意外的是，作为一种泛素样蛋白（ubiquitin-like protein）的ATG12本身可被直接泛素化，而该修饰会促进其经蛋白酶体途径降解。就其周转代谢的功能后果而言，游离态ATG12的积累会加剧蛋白酶体抑制剂诱导的细胞凋亡——鉴于蛋白酶体抑制剂已被用作抗癌药物，这一发现或具有重要的临床价值。综上，我们的研究结果揭示了自噬、蛋白酶体活性与细胞死亡之间一种全新的关联，该关联由ATG12的泛素样特性所介导。