Polymer Theory of Biologically Functional Phase Separations of Intrinsically Disordered Proteins

目前的聚合物理论主要与均聚物有关。对杂多聚合物的关注则少得多。这种相对的忽视不仅是因为杂多聚合物对理论更具挑战性，也反映了可用于对此类理论进行基准测试的有限的实验数据，因为在化学合成具有特定序列的杂多聚合物方面存在困难。因此，对于杂化聚合物的分析理论大多局限于处理无规或简单的嵌段共聚物。相反，这一限制不适用于分子生物学，在分子生物学中，蛋白质和核酸序列的准确合成是例行公事。在这方面，最近发现的本征无序蛋白质(IDP)相分离在活细胞内和周围的各种凝聚的液体/凝胶样体/组织(包括无膜细胞器，如应力颗粒和核仁)中的关键和普遍的作用，构成了重要的推动力，并为发展序列相关的杂聚相性质的聚合物理论提供了令人兴奋的前景。 尽管观察到境内流离失所者的阶段分离引起了人们的强烈兴趣，但对这一现象的实际了解还处于初级阶段。平均场Flory-Huggins(FH)和Overbeek-Voorn理论已经被引用，但它们本身是不够的，因为缺乏对序列特异性的解释。在此背景下，申请人的团队取得了第一个突破，应用随机相近似(RPA)聚合物理论来解决聚两性离子的相行为如何依赖于其链序列上的电荷模式。我们进一步证明，中性聚两性阳离子在多链环境中相分离的倾向与其作为单链的构象尺寸呈强烈的负相关。最近，使用RPA的扩展，我们还发现两个IDP序列的链在相分离时混合或分离的趋势与它们的电荷模式的差异有关。 在这些开创性进展的基础上，我们将进一步开发分析公式，不仅包括静电学，还包括其他形式的单体间相互作用以及链硬度的局部变化的序列依赖效应。为了更好地解释具有高净电荷密度和与短空间范围(如疏水性)相互作用的IDPs，我们将设计新的技术来应用重整化Kuhn长度的变分方法来绕过RPA理论的某些缺陷。通过使用我们的理论来合理化快速增长的IDP相分离实验数据并做出可测试的预测，所提出的努力将为细胞划分提供有价值的见解，并也可以促进新材料的开发。

Polymer theories as they stand are mainly concerned with homopolymers. Considerably less attention has been paid to heteropolymers. This relative neglect is not only because heteropolymers are more challenging for theory; it has also been a reflection of the limited available experimental data for benchmarking such theories due to difficulties in chemically synthesizing heteropolymers with specific sequences. Consequently, analytical theories for heteropolymers were mostly confined to treating random or simple block copolymers. In contrast, this limitation does not apply to molecular biology wherein accurate synthesis of protein and nucleic acid sequences is routine. In this respect, the recent discovery of the critical and ubiquitous roles of intrinsically disordered protein (IDP) phase separation in a variety of condensed liquid/gel-like bodies/organizations in and around living cells (including membraneless organelles such as stress granules and the nucleolus) constitutes an important impetus and offers exciting prospects for developing polymer theories for sequence-dependent heteropolymer phase properties. Despite the intense interest generated by the observation of IDP phase separations, physical understanding of the phenomenon is only at its infancy. Mean-field Flory-Huggins (FH) and Overbeek-Voorn theories have been invoked, but they alone are insufficient because an account for sequence-specificity is lacking. In this context, the applicant's group made the first breakthrough by applying the random-phase approximation (RPA) polymer theory to address how phase behaviors of polyampholytes depend on the charge patterns along their chain sequences. We showed further that the propensity for a neutral polyampholyte to phase separate in a multiple-chain setting is strongly and negatively correlated with its conformational dimensions as an individual chain. Most recently, using an extension of RPA, we found also that the tendency for chains of two IDP sequences to mix or demix upon phase separation is correlated with the difference in their charge patterns. Building on these seminal advances, we will develop further analytical formulations for sequence-dependent effects of not only electrostatics but also other forms of inter-monomer interactions as well as local variations of chain stiffness. In order to afford a better account of IDPs with high net charge densities and interactions with short spatial ranges such as hydrophobicity, we will device novel techniques to apply variational methods for renormalized Kuhn lengths to circumvent certain shortcomings of RPA theories. By using our theory to rationalize the rapidly expanding repertoire of experimental data on IDP phase separation and to make testable predictions, the proposed effort will provide valuable insights into cellular compartmentalization and can also facilitate development of new materials.