Cold-Induced Changes in the Protein Ubiquitin

Conformational changes are essential for protein-protein and protein-ligand recognition. Here we probed changes in the structure of the protein ubiquitin at low temperatures in supercooled water using NMR spectroscopy. We demonstrate that ubiquitin is well folded down to 263 K, although slight rearrangements in the hydrophobic core occur. However, amide proton chemical shifts show non-linear temperature dependence in supercooled solution and backbone hydrogen bonds become weaker in the region that is most prone to cold-denaturation. Our data suggest that the weakening of the hydrogen bonds in the β-sheet of ubiquitin might be one of the first events that occur during cold-denaturation of ubiquitin. Interestingly, the same region is strongly involved in ubiquitin-protein complexes suggesting that this part of ubiquitin more easily adjusts to conformational changes required for complex formation.构象变化是蛋白质-蛋白质与蛋白质-配体识别过程的关键要素。本研究采用核磁共振波谱法（NMR spectroscopy），对过冷水中低温环境下泛素（ubiquitin）的结构变化进行了探究。实验结果显示，泛素在低至263 K的温度下仍保持良好折叠状态，仅其疏水核心发生了轻微的构象重排。然而，在过冷溶液中，酰胺质子的化学位移呈现出非线性的温度依赖性；且最易发生冷变性（cold-denaturation）的区域内，主链氢键逐渐减弱。本研究数据表明，泛素β折叠片内氢键的弱化，可能是泛素冷变性过程中最早出现的事件之一。值得关注的是，该区域同样广泛参与泛素-蛋白质复合物的形成，这提示泛素的该区域可更便捷地适配复合物形成所需的构象变化。