Department of Biochemistry and Molecular Biophysics, Washington University in St. Louis, St. Louis, Missouri 63110, USA.
Department of Biochemistry, University of Zurich, Zurich 8057, Switzerland.
J Chem Phys. 2018 Mar 28;148(12):123326. doi: 10.1063/1.5009286.
Internal friction is frequently found in protein dynamics. Its molecular origin however is difficult to conceptualize. Even unfolded and intrinsically disordered polypeptide chains exhibit signs of internal friction despite their enormous solvent accessibility. Here, we compare four polymer theories of internal friction with experimental results on the intrinsically disordered protein ACTR (activator of thyroid hormone receptor). Using nanosecond fluorescence correlation spectroscopy combined with single-molecule Förster resonance energy transfer (smFRET), we determine the time scales of the diffusive chain dynamics of ACTR at different solvent viscosities and varying degrees of compaction. Despite pronounced differences between the theories, we find that all models can capture the experimental viscosity-dependence of the chain relaxation time. In contrast, the observed slowdown upon chain collapse of ACTR is not captured by any of the theories and a mechanistic link between chain dimension and internal friction is still missing, implying that the current theories are incomplete. In addition, a discrepancy between early results on homopolymer solutions and recent single-molecule experiments on unfolded and disordered proteins suggests that internal friction is likely to be a composite phenomenon caused by a variety of processes.
内摩擦在蛋白质动力学中经常被发现。然而,其分子起源很难理解。即使是展开的和固有无序的多肽链也表现出内摩擦的迹象,尽管它们具有巨大的溶剂可及性。在这里,我们将比较四种聚合物内摩擦理论与内在无序蛋白 ACTR(甲状腺激素受体激活蛋白)的实验结果。我们使用纳秒荧光相关光谱学结合单分子Förster 共振能量转移(smFRET),在不同溶剂粘度和不同程度的压缩下,确定 ACTR 扩散链动力学的时间尺度。尽管理论之间存在明显差异,但我们发现所有模型都可以捕捉到链弛豫时间对粘度的实验依赖性。相比之下,ACTR 链坍塌时观察到的减速现象并没有被任何理论所捕捉到,并且链尺寸和内摩擦之间的机械联系仍然缺失,这意味着目前的理论是不完整的。此外,在均聚物溶液的早期结果和最近对展开和无序蛋白质的单分子实验之间存在差异,这表明内摩擦可能是由多种过程引起的综合现象。