Abstract：
Membrane protein folding is distinct from folding of soluble proteins. Conformational acquisition in major membrane protein subclasses can be delineated into insertion and folding processes. An exception to the \"two stage\" folding, later developed to \"three stage\" folding, is observed within the last two helices in bacteriorhodopsin (BR), a system that serves as a model membrane protein. We employ a reductionist approach to understand interplay of molecular factors underlying the apparent defiance. Leveraging available solution NMR structures, we construct, sample in silico, and analyze partially (PIn) and fully inserted (FIn) BR membrane states. The membrane lateral C-terminal helix (CH) in PIn is markedly prone to transient structural distortions over microsecond timescales; a disorder prone region (DPR) is thereby identified. While clear transmembrane propensities are not acquired, the distortions induce alterations in local membrane curvature and area per lipid. Importantly, energetic decompositions reveal that overall, the N-terminal helix (NH) is thermodynamically more stable in the PIn. Higher overall stability of the FIn arises from favorable interactions between the NH and the CH. Our results establish lack of spontaneous transition of the PIn to the FIn, and attributes their partitioning to barriers that exceed those accessible with thermal fluctuations. This work paves the way for further detailed studies aimed at determining the thermo-kinetic roles of the initial five helices, or complementary external factors, in complete helical folding and insertion in BR. We comment that complementing such efforts with the growing field of machine learning assisted energy landscape searches may offer unprecedented insights.
摘要：
膜蛋白折叠不同于可溶性蛋白的折叠。主要膜蛋白亚类中的构象获得可以描述为插入和折叠过程。“两阶段”折叠的例外，后来发展到“三级”折叠，在细菌视紫红质（BR）的最后两个螺旋中观察到，作为膜蛋白模型的系统。我们采用简化主义的方法来理解明显挑战背后的分子因素的相互作用。利用可用的解决方案NMR结构，我们建造，硅片中的样品，并分析部分（PIn）和完全插入（FIn）BR膜状态。PIn中的膜横向C末端螺旋（CH）明显容易在微秒的时间尺度上发生瞬时结构畸变；从而鉴定出易发疾病区域（DPR）。虽然没有获得明显的跨膜倾向，扭曲会引起局部膜曲率和每个脂质面积的变化。重要的是,能量分解揭示了总体上，N-末端螺旋(NH)在Pin中热力学上更稳定。FIn的较高总体稳定性源于NH和CH之间的有利相互作用。我们的结果确定缺乏Pin到Fin的自发过渡，并将其分区归因于超过热波动可达到的屏障。这项工作为旨在确定初始五个螺旋的热动力学作用的进一步详细研究铺平了道路。或互补的外部因素，在完全螺旋折叠和插入BR。我们评论说，用不断增长的机器学习辅助能源景观搜索领域来补充这种努力可能会提供前所未有的见解。
