Cellular Functions of Proteasome-Associated Ubiquitin Ligase Activity

泛素- 26s蛋白酶体通过选择性降解短寿命或受损的蛋白质，在细胞信号传导中起着至关重要的作用。在真核生物中，保守的小蛋白泛素的附着标志着底物被招募到蛋白酶体，在那里它们被蛋白水解成小肽。不能降解泛素标记的底物会导致严重的细胞应激，是不同真核生物发育缺陷的主要原因，包括神经退行性疾病、自身免疫、心肌病和囊性纤维化等遗传性疾病等人类疾病。因此，了解控制健康蛋白酶体功能的保守调节机制有可能影响生物医学和生物技术的各个领域。最近的研究结果表明，到达蛋白酶体后，底物可能会被泛素进一步修饰。但是，如果底物已经被招募到蛋白酶体上，为什么还需要进一步的多泛素化呢？尽管有人认为这种“最后一刻”的多泛素化促进了蛋白酶体的加工，但支撑这种活动的机制仍然难以捉摸。我们最近证明，来自hect型家族的泛素连接酶与蛋白酶体物理关联，从而控制普遍底物多泛素化和相关的表型特征。有趣的是，hect型连接酶与来自不同信号通路的多种E3泛素连接酶相互作用。基于这些发现，我们建议阐明hect型连接酶的底物库，并探索其在信号传导中的作用的新假设：(1)健康的蛋白酶体功能是否需要从多个E3连接酶到蛋白酶体相关的hect型连接酶的底物中转？(2)蛋白酶体相关的hect型连接酶是否会改变相互作用的E3连接酶的活性，从而使蛋白酶体能够反馈调节泛素介导的信号通路？了解hect型连接酶如何促进蛋白酶体功能将有助于有针对性地设计新策略，以改善疾病期间蛋白酶体的功能。

The ubiquitin-26S proteasome has vital roles in cellular signalling by selectively degrading proteins that are short-lived or damaged. In eukaryotes attachment of the conserved small protein ubiquitin marks substrates for recruitment to the proteasome where they are proteolysed into small peptides. Failure to degrade ubiquitin-marked substrates causes severe cellular stress and is a leading cause of developmental defects across different eukaryotes, including human pathologies such as neurodegenerative diseases, autoimmunity, cardiomyopathy, and genetic disorders like cystic fibrosis. Understanding the conserved regulatory mechanisms that govern healthy proteasome functioning therefore has the potential to impact diverse fields in biomedicine and biotechnology. Recent findings indicate that upon arrival at the proteasome, substrates may undergo further modification by ubiquitin. But why do substrates require further polyubiquitination if they have already been recruited to the proteasome? Although it has been suggested that such 'last-minute' polyubiquitination facilitates proteasome processivity, the mechanisms underpinning this activity remain elusive. We recently demonstrated that ubiquitin ligases from the HECT-type family physically associate with the proteasome, thereby governing universal substrate polyubiquitination and associated phenotypical traits. Intriguingly, HECT-type ligases interact with multiple E3 ubiquitin ligases from distinct signalling pathways. Based on these findings we propose to elucidate the substrate repertoires of HECT-type ligases and explore new hypotheses for their role in signalling: (1) does healthy proteasome function require substrate relay from multiple E3 ligases to proteasome-associated HECT-type ligases?; (2) do proteasome-associated HECT-type ligases modify the activity of interacting E3 ligases, thereby enabling proteasomes to feedback regulate ubiquitin-mediated signalling pathways? Understanding how HECT-type ligases promote proteasome function will aid in targeted design of new strategies to improve proteasome function during disease.