Cellular Functions of Proteasome-Associated Ubiquitin Ligase Activity

泛素-26S蛋白酶体通过选择性地降解短命或受损的蛋白质，在细胞信号转导中发挥重要作用。在真核生物中，保守的小蛋白泛素的附着标记底物，使其重新聚集到蛋白酶体中，在那里它们被蛋白质分解成小肽。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，包括人类病理，如神经退行性疾病、自身免疫、心肌病和遗传性疾病，如囊性纤维化。因此，了解管理健康蛋白酶体功能的保守调控机制有可能影响生物医学和生物技术的各个领域。最近的发现表明，当到达蛋白酶体时，底物可能会受到泛素的进一步修饰。但是，如果底物已经被招募到蛋白酶体中，为什么它们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，但支持这一活动的机制仍然不清楚。我们最近证明了来自Hect-type家族的泛素连接酶在物理上与蛋白酶体结合，从而控制普遍的底物多泛素化和相关的表型特征。有趣的是，Hect-type连接酶从不同的信号通路与多个E3泛素连接酶相互作用。基于这些发现，我们建议阐明Hect-type连接酶的底物谱系，并探索它们在信号传递中的作用的新假设：(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-type连接酶的底物传递？(2)蛋白酶体相关的Hect-type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-type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的策略，以改善疾病期间的蛋白酶体功能。

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.