Challenging the ubiquitin proteolytic signal - Novel modes of modification and recognition

26S蛋白酶体复合体识别的“典型”蛋白水解性信号是基于解氨酸48的多泛素链。然而，最近的证据表明，蛋白酶体可以识别单素底物以及具有不同内部连接的泛素链。此外，似乎即使是“典型的”泛素化级联E1-E2-E3也没有解释信号在长度、连接类型和位置上的多样性，也没有解释额外的辅助因素和重塑的要求。例如，该联盟的成员发现：(I)单素化促进转录激活剂活性p50亚基的p105 NF-KB前体的有效蛋白酶体处理；(Ii)单个E3连接酶Hrd1利用三种不同的泛素结合的E2酶-Ubc1、6和7，通过ER相关降解(ERAD)调节内质网(ER)的质量控制；(Iii)Aaxin 3的多谷氨酰胺扩张导致严重形式的共济失调，使其DuB活性失活；表明酶的编辑/链缩短功能在疾病中发挥了致病作用；(4)单素化是细胞中确定的主要修饰。最令人惊讶的是，通过过度表达非聚合泛素来干扰修饰信号并没有阻碍细胞生理学。在酵母、线虫和哺乳动物中使用分子、生化、遗传、细胞和有机学的组合方法，我们将绘制这些新信号的利用程度，它们的识别和靶向机制，以及它们的病理生理学意义。

The "canonical" proteolytic signal that is recognized by the 26S proteasome complex is alysine 48-based polyubiquitin chain. However recent evidence suggests that the proteasome can recognize monoubiquitinated substrates as well as ubiquitin chains with different internallinkages. Furthermore, it appears that even the "canonical" ubiquitination cascade E1-E2-E3explains neither the diversity of the signal in length and linkage type and site, nor therequirements for additional ancillary factors and remodeling. Thus for example, members ofthis consortium found that; (i) monoubiquitination promotes efficient proteasomal processing ofthe p105 NF-KB precursor to the active p50 subunit of the transcriptional activator; (ii) a singleE3 ligase, Hrd1, utilizes three different ubiquitin-conjugating E2 enzymes – Ubc1, 6, and 7, toregulate quality control in the endoplasmic reticulum (ER) via ER-associated degradation(ERAD); (iii) the poly-glutamine expansion of Ataxin 3 which leads to a severe form of ataxia,inactivates its DUB activity suggests that the editing/chain shortening function of the enzymeplays a pathogenetic role in the disease; (iv) monoubiquitination is the predominantmodification identified in the cell. Most surprisingly, perturbing the modifying signal byoverexpression of non-polymerizable ubiquitin did not hamper cellular physiology. Using acombined molecular, biochemical, genetic, cellular, and organismal approach in yeast,nematode, and mammals, we shall map the extent of utilization of these novel signals, their mechanisms of recognition and targeting, and their pathophysiological implications.