泛素-蛋白酶体通路与自噬-溶酶体途径是细胞内蛋白质降解的主要途径。蛋白酶体激活因子 PA28γ可以与 20S 催化颗粒结合形成 PA28γ蛋白酶体，以非泛素依赖的方式催化蛋白质的降解，在细胞周期、凋亡等过程中起着十分关键的调控作用。哺乳动物细胞中，LC3 蛋白调控自噬体的形成以及自噬底物的识别。至少存在 7 种亚家族成员，包括：LC3A、LC3B、LC3B2、LC3C、 GABARAP、GABARAPL1 和 GABARAPL2。其中，LC3B（本论文统称为 LC3）是最重要的LC3-I 和 LC3-II 的前体。我们实验室前期的工作表明，PA28γ蛋白酶体可降解LC3-I，而凋亡抑制蛋白 BRUCE/BIRC6 促进 LC3-I 的降解。同时，BRUCE 可以作为 E2（泛素耦合酶） 和 E3（泛素连接酶）催化 LC3 的 K51 泛素化位点。本论文发现，LC3 的泛素化位点突变（K51R）导致其自身由 PA28γ介导的降解明显受到抑制。进一步通过免疫共沉淀实验发现，LC3-K51R 仍可以与 PA28γ和BRUCE 结合，说明 LC3 泛素化并不是 PA28γ底物识别所必需。LC3 在自噬小体形成中也起着关键作用，但 LC3-K51R 突变并不妨碍其转运到高尔基体及后续的自噬小体中。重要的是，LC3 的 K51R 突变可以促进雷帕霉素（Rapamycin）诱导的自噬从而加快另一自噬底物受体蛋白 p62 以及 BRUCE 通过自噬降解，并抑制拓扑异构酶抑制剂 Etoposide 诱导的细胞凋亡。这些结果表明，LC3-K51 位点泛素化调节细胞自噬与凋亡之间的相互平衡。本研究对于肿瘤、神经退行性疾病等自噬和凋亡相关疾病的研究具有潜在的指导意义。

The Ubiquitin-proteasome pathway and Autophagy-lysosomal pathway are the main pathways for intracellular protein degradation. The proteasomal activator PA28γ combines with 20S catalytic particles to form PA28γ-proteasome, which catalyzes the degradation of protein and plays a key role in the regulation of cell cycle and apoptosis in a ubiquitin-independent manner. In mammalian cells, LC3 protein regulates the formation of autophagosomes and the recognition of autophagic substrates, including LC3A, LC3B, LC3B2, LC3C, GABARAP, GABARAPL1 and GABARAPL2. LC3B (collectively referred to as LC3 in this paper) is the precursor of LC3-I and LC3-II. Our previous work shows that PA28γ-proteasome promotes the degradation of LC3-I. While the inhibitor of apoptosis protein, BRUCE / BIRC6 promotes the degradation of LC3-I, which acts as E2 (ubiquitin coupling enzyme) and E3 (ubiquitin ligase) of LC3 via polyubiquitinating it at the K51 ubiquitination site. In this paper, it is found that the PA28γ-proteasome mediated degradation of LC3 was significantly inhibited after mutating the ubiquitination site, K51 into K51R. Further, LC3-K51R could still interact with PA28γ-proteasome and BRUCE, indicating that the ubiquitination of LC3 is not necessary for the substrate recognition of PA28γ-proteasome. LC3 also plays a key role in the formation of autophagosomes, but LC3-K51R mutation can not prevent its translocating into trans-Golgi network and autophagosomes. LC3-K51R mutation promotes rapamycin induced autophagy, thus accelerates the autophagic degradation of autophagy receptor protein p62 and apoptosis inhibitory protein BRUCE, and inhibits the topoisomerase inhibitor etoposide induced apoptosis. This study suggests that the ubiquitination of LC3 at K51 site regulates the balance between autophagy and apoptosis, which has potential significance for the study of autophagy and apoptosis related diseases, such as tumor, and neurodegenerative diseases.