泛素蛋白磷酸化修饰的功能与解析

Functions of Phosphorylated Ubiquitin

泛素化是存在于真核生物中一种重要的翻译后修饰过程,参与调控包括蛋白质降解在内的多种生命活动。实现这一调控过程需要将一个由76个氨基酸组成的泛素蛋白共价连接到底物蛋白上。同时,泛素本身也存在多种翻译后修饰,包括泛素化、磷酸化、乙酰化等,进一步丰富了泛素的修饰类型,决定了底物蛋白不同的命运。近年来,伴随着第65位丝氨酸磷酸化泛素蛋白参与调控线粒体自噬这一突破性进展,泛素蛋白其余磷酸化位点的功能研究也获得越来越多的关注。本文根据目前已有的国内外研究和报道,总结了泛素蛋白已知的磷酸化修饰位点,梳理了泛素蛋白第12位和66位苏氨酸、第57位和65位丝氨酸等位点的磷酸化修饰对其生物物理特性带来的改变,并对相应修饰位点所涉及的生物学功能调控进行了综述。

Ubiquitylation,also termed ubiquitination,is one of the most important post-translational modifications in eukaryotic cells.It is a process by which a small signaling protein,called ubiquitin composed of 76 amino acids,is conjugated to protein substrates via an E1-E2-E3 enzymatic cascade.Ubiquitin can be attached to lysine,serine,threonine and cysteine residues of its substrates.Ubiquitin itself contains seven lysine residues,therefore,ubiquitylation can form various polyubiquitin chains to produce complex ubiquitin codes.Ubiquitylation can alter the fates of ubiquitylated proteins including kinase activation,alteration of protein localization and proteolysis via the 26S proteasome and is involved in nearly every aspect of biological activities in eukaryotic cells.Recent studies indicated that more complicated post-translational modifications can also be found on ubiquitin including sumoylation,ubiquitylation,phosphorylation and acetylation.These modifications largely increase the complexity of ubiquitin signals.Ser65 of ubiquitin is the first characterized phosphorylation site whose biological functions have been extensively studied in human cells.It has been shown that Ser65 phosphorylation by PINK1 kinase is critical for the activation of Parkin ubiquitin ligase during mitophagy induction.The researches on Ser65 phosphorylation of ubiquitin boosted the studies on biological significance of the rest phosphorylation sites of ubiquitin.Now it is clear that phosphorylation of Ser57 residue is involved in endocytosis and stress responses,including oxidative stress in yeast,whereas phosphorylation of Thr12 and Thr66 residues plays important roles in DNA damage response.In the case of Ser57 residue,members of the AMPKrelated kinases phosphorylate it,however,mechanisms by which Ser57 phosphorylation regulates endocytosis or oxidative stress response are still unclear.Also,no experimental evidences are available in mammalian system yet.One interesting fact is that many AMPK-related kinases contain a ubiquitin-associated domain(UBA),although some studies suggested these UBA domains do not possess any binding capabilities to polyubiquitin chains.However,some of these kinases could phosphorylate the Ser57 residues on the M1 polyubiquitin chain,implying these UBA domains do recognize certain polyubiquitin chains.In this review article,we summarized the post-translational modification sites of ubiquitin,especially phosphorylation sites and highlighted the biological functions of Ser65,Ser57,Thr12,Thr66 phospho-ubiquitin proteins.We also discussed alternations of biophysical properties brought by the phosphorylation of ubiquitin.Finally,we proposed a few future research directions related to the phosphorylated ubiquitin.