Small ubiquitin-related modifiers (SUMO1 and SUMO2) are ubiquitin family proteins, structurally similar to ubiquitin, differing in terms of their amino acid sequence and functions. Therefore, they provide a great platform for investigating sequence-structure-stability-function relationship. Here, we used chemical denaturation in comparing the folding-unfolding pathways of the SUMO proteins with their structural homologue ubiquitin (UF45W-pseudo wild-type [WT] tryptophan variant) with structurally analogous tryptophan mutations (SUMO1 [S1F66W], SUMO2 [S2F62W]). Equilibrium denaturation studies report that ubiquitin is the most stable protein among the three. The observed denaturant-dependent folding rates of SUMOs are much lower than ubiquitin and primarily exhibit a two-state folding pathway unlike ubiquitin, which has a kinetic folding intermediate. We hypothesize that, as SUMO proteins start off as slow folders, they avoid stabilizing their folding intermediates and the presence of which might further slow-down their folding rates. The denaturant-dependent unfolding of ubiquitin is the fastest, followed by SUMO2, and slowest for SUMO1. However, the spontaneous unfolding rate constant is the lowest for ubiquitin (~40 times), and similar for SUMOs. This correlation between thermodynamic stability and kinetic stability is achieved by having different unfolding transition state positions with respect to the solvent-accessible surface area, as quantified by the Tanford β u values: ubiquitin (0.42) > SUMO2 (0.20) > SUMO1 (0.16). The results presented here highlight the unique energy landscape features which help in optimizing the folding-unfolding rates within a structurally homologous protein family.

中文翻译：

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泛素家族蛋白能量构象特征的实验比较。

小型泛素相关修饰剂（SUMO1和SUMO2）是泛素家族蛋白，在结构上与泛素相似，但氨基酸序列和功能不同。因此，它们为研究序列-结构-稳定性-功能关系提供了一个很好的平台。在这里，我们使用化学变性比较了SUMO蛋白及其结构同源泛素（UF45W-假野生型[WT]色氨酸变体）与结构类似色氨酸突变（SUMO1 [S1F66W]，SUMO2 [S2F62W]）的折叠-展开途径）。平衡变性研究报告，泛素是这三种蛋白中最稳定的蛋白。观察到的SUMOs依赖于变性剂的折叠速率远低于泛素，并且与泛素不同，它主要表现出一种二态折叠途径，具有动力学折叠中间体。我们假设，由于SUMO蛋白以缓慢的折叠开始，因此它们避免了稳定其折叠中间体，并且它们的存在可能会进一步减慢其折叠速率。泛素的依赖于变性剂的展开是最快的，其次是SUMO2，对于SUMO1是最慢的。但是，自发展开速率常数对于泛素最低（约40倍），而对于SUMO相似。热力学稳定性和动力学稳定性之间的这种关系是通过相对于溶剂可及的表面积具有不同的展开过渡状态位置来实现的，如Tanfordβu值所定量：泛素（0.42）> SUMO2（0.20）> SUMO1（0.16） 。