National Institute for Medical Research, Medical Research Council London, UK.
Medical Research Council Biomedical NMR Centre, National Institute for Medical Research, Medical Research Council London, UK.
Front Mol Biosci. 2015 Feb 5;2:2. doi: 10.3389/fmolb.2015.00002. eCollection 2015.
Ataxin-3, the protein responsible for spinocerebellar ataxia type-3, is a cysteine protease that specifically cleaves poly-ubiquitin chains and participates in the ubiquitin proteasome pathway. The enzymatic activity resides in the N-terminal Josephin domain. An unusual feature of ataxin-3 is its low enzymatic activity especially for mono-ubiquitinated substrates and short ubiquitin chains. However, specific ubiquitination at lysine 117 in the Josephin domain activates ataxin-3 through an unknown mechanism. Here, we investigate the effects of K117 ubiquitination on the structure and enzymatic activity of the protein. We show that covalently linked ubiquitin rests on the Josephin domain, forming a compact globular moiety and occupying a ubiquitin binding site previously thought to be essential for substrate recognition. In doing so, ubiquitination enhances enzymatic activity by locking the enzyme in an activated state. Our results indicate that ubiquitin functions both as a substrate and as an allosteric regulatory factor. We provide a novel example in which a conformational switch controls the activity of an enzyme that mediates deubiquitination.
三核苷酸重复疾病相关蛋白 3（ataxin-3）是一种半胱氨酸蛋白酶，负责引起脊髓小脑共济失调 3 型，可特异性切割多聚泛素链并参与泛素蛋白酶体途径。该酶的活性位于 N 端的 Josephin 结构域。ataxin-3 的一个显著特征是其酶活性较低，尤其是对单泛素化底物和短泛素链。然而，Josephin 结构域中赖氨酸 117 上的特异性泛素化通过未知机制激活 ataxin-3。在这里，我们研究了 K117 泛素化对蛋白结构和酶活性的影响。我们发现，共价连接的泛素位于 Josephin 结构域上，形成一个紧凑的球状结构，并占据了先前被认为对底物识别至关重要的泛素结合位点。这样，泛素化通过将酶锁定在激活状态来增强酶活性。我们的结果表明，泛素既作为底物又作为别构调节因子发挥作用。我们提供了一个新的例子，其中构象开关控制介导去泛素化的酶的活性。
