近日，美国PLoS ONE杂志发表了中科院上海生命科学研究院生化与细胞所胡红雨课题组的研究论文。该论文阐述了III型脊髓小脑共济失调症相关蛋白ataxin-3（AT3）中的串联泛素结合模体（ubiquitin-interaction motif，UIM）的构象变化和UIM与泛素链结合的协同效应。
UIM是一类拥有泛素结合和促进泛素化功能的短肽序列。UIM存在于许多不同类型的蛋白质中，其氨基酸序列高度保守。除了与单泛素（mono-Ub）结合外，UIM还能够与多泛素链（poly-Ub）结合。AT3是一种含有多聚谷氨酰胺（polyQ）的蛋白质，其polyQ附近有两个相连的UIM序列（UIM12），对于结合泛素和促进底物泛素化有着十分重要的作用。
宋爱新博士等人利用核磁共振的方法，解析了UIM12在自由状态和结合泛素状态下的溶液结构，每一个UIM模体都由一个a-螺旋组成。当UIM12处于自由状态时，这两个螺旋之间没有固定的取向，而当结合了泛素后，这两个螺旋便形成了典型的螺旋－环－螺旋（helix-loop-helix）的折叠构象。核磁共振动态学数据也显示，当UIM12结合了泛素后，两个螺旋间的连接部分变得更为刚性。通过核磁共振化学位移干扰（CSP）和等温滴定量热（ITC）实验，发现UIM12比单独的UIM1、UIM2与泛素的结合力更强；而与结合单泛素相比，UIM12更倾向于结合通过K48或K63连接的二泛素链，并且两个UIM之间的连接部分对于UIM12与二泛素链的结合起着重要作用。
这些证据表明，UIM12的连接区域通过构象和动态性质的变化，使两个UIM模体以协同效应的方式与单泛素或二泛素链结合，这也许与蛋白质识别不同类型的多泛素链和泛素化底物有关。
该项研究与上海药物所林东海课题组合作完成，工作得到了国家科技部、基金委、中国科学院的经费支持。（生物谷Bioon.com）
生物谷推荐英文摘要：
PLoS ONE 5(10): e13202. doi:10.1371/journal.pone.0013202
Structural Transformation of the Tandem Ubiquitin-Interacting Motifs in Ataxin-3 and Their Cooperative Interactions with Ubiquitin Chains
Ai-Xin Song1, Chen-Jie Zhou1, Yu Peng2, Xue-Chao Gao1, Zi-Ren Zhou1, Qing-Shan Fu1, Jing Hong2, Dong-Hai Lin2*, Hong-Yu Hu1*
1 State Key Laboratory of Molecular Biology, Institute of Biochemistry and Cell Biology, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, Shanghai, China, 2 Shanghai Institute of Materia Medica, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, Shanghai, China
The ubiquitin-interacting motif (UIM) is a short peptide with dual function of binding ubiquitin (Ub) and promoting ubiquitination. We elucidated the structures and dynamics of the tandem UIMs of ataxin-3 (AT3-UIM12) both in free and Ub-bound forms. The solution structure of free AT3-UIM12 consists of two α-helices and a flexible linker, whereas that of the Ub-bound form is much more compact with hydrophobic contacts between the two helices. NMR dynamics indicates that the flexible linker becomes rigid when AT3-UIM12 binds with Ub. Isothermal titration calorimetry and NMR titration demonstrate that AT3-UIM12 binds diUb with two distinct affinities, and the linker plays a critical role in association of the two helices in diUb binding. These results provide an implication that the tandem UIM12 interacts with Ub or diUb in a cooperative manner through an allosteric effect and dynamics change of the linker region, which might be related to its recognitions with various Ub chains and ubiquitinated substrates.
