Max Planck Institute for Medical Research, Department of Biomolecular Mechanismsm Heidelberg, Germany.
Karlsruhe Institute of Technology, Steinbuch Centre for Computing and Department of Physics, Eggenstein-Leopoldshafen, Germany.
J Mol Biol. 2021 Apr 2;433(7):166859. doi: 10.1016/j.jmb.2021.166859. Epub 2021 Feb 1.
Icosahedral viral capsids assemble with high fidelity from a large number of identical buildings blocks. The mechanisms that enable individual capsid proteins to form stable oligomeric units (capsomers) while affording structural adaptability required for further assembly into capsids are mostly unknown. Understanding these mechanisms requires knowledge of the capsomers' dynamics, especially for viruses where no additional helper proteins are needed during capsid assembly like for the Mavirus virophage that despite its complexity (triangulation number T = 27) can assemble from its major capsid protein (MCP) alone. This protein forms the basic building block of the capsid namely a trimer (MCP) of double-jelly roll protomers with highly intertwined N-terminal arms of each protomer wrapping around the other two at the base of the capsomer, secured by a clasp that is formed by part of the C-terminus. Probing the dynamics of the capsomer with HDX mass spectrometry we observed differences in conformational flexibility between functional elements of the MCP trimer. While the N-terminal arm and clasp regions show above average deuterium incorporation, the two jelly-roll units in each protomer also differ in their structural plasticity, which might be needed for efficient assembly. Assessing the role of the N-terminal arm in maintaining capsomer stability showed that its detachment is required for capsomer dissociation, constituting a barrier towards capsomer monomerisation. Surprisingly, capsomer dissociation was irreversible since it was followed by a global structural rearrangement of the protomers as indicated by computational studies showing a rearrangement of the N-terminus blocking part of the capsomer forming interface.
二十面体病毒衣壳由大量相同的结构单元以高度保真的方式组装而成。使单个衣壳蛋白形成稳定的寡聚体单元（衣壳粒），同时赋予进一步组装成衣壳所需的结构适应性的机制在很大程度上尚不清楚。了解这些机制需要了解衣壳粒的动力学特性，特别是对于那些在衣壳组装过程中不需要额外辅助蛋白的病毒，例如 Mavirus 噬菌体，尽管它的结构非常复杂（三角数 T=27），但它可以仅由其主要衣壳蛋白（MCP）组装而成。这种蛋白质构成了衣壳的基本构建块，即三聚体（MCP）的双果冻卷原聚体，每个原聚体的高度交织的 N 端臂包裹在衣壳粒底部的另外两个原聚体上，由 C 端的一部分形成的扣环固定。我们通过 HDX 质谱法探测衣壳粒的动力学，观察到 MCP 三聚体的功能元件之间的构象灵活性存在差异。虽然 N 端臂和扣环区域的氘掺入率高于平均水平，但每个原聚体中的两个果冻卷单元在结构可塑性上也存在差异，这可能是有效组装所必需的。评估 N 端臂在维持衣壳粒稳定性方面的作用表明，其脱离是衣壳粒解离所必需的，构成了衣壳粒单体化的障碍。令人惊讶的是，衣壳粒解离是不可逆的，因为它随后伴随着原聚体的全局结构重排，这一点通过计算研究表明，原聚体的 N 端发生了重排，从而阻止了衣壳粒形成界面的部分区域。
