Proteins that self-assemble into regular shell-like polyhedra are useful, both in nature and in the laboratory, as molecular containers. Here we describe cryo-electron microscopy (EM) structures of two versatile encapsulation systems that exploit engineered electrostatic interactions for cargo loading. We show that increasing the number of negative charges on the lumenal surface of lumazine synthase, a protein that naturally assembles into a ∼1-MDa dodecahedron composed of 12 pentamers, induces stepwise expansion of the native protein shell, giving rise to thermostable ∼3-MDa and ∼6-MDa assemblies containing 180 and 360 subunits, respectively. Remarkably, these expanded particles assume unprecedented tetrahedrally and icosahedrally symmetric structures constructed entirely from pentameric units. Large keyhole-shaped pores in the shell, not present in the wild-type capsid, enable diffusion-limited encapsulation of complementarily charged guests. The structures of these supercharged assemblies demonstrate how programmed electrostatic effects can be effectively harnessed to tailor the architecture and properties of protein cages.

中文翻译：

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可扩展多孔蛋白质笼的结构和组装。


自组装成规则的壳状多面体的蛋白质作为分子容器在自然界和实验室中都很有用。在这里，我们描述了两种多功能封装系统的冷冻电子显微镜（EM）结构，它们利用工程静电相互作用来装载货物。我们发现，增加二氢四氢呋喃合成酶（一种自然组装成由 12 个五聚体组成的~1-MDa 十二面体的蛋白质）管腔表面的负电荷数量，会诱导天然蛋白质壳的逐步膨胀，从而产生热稳定性~3- MDa 和~6-MDa 组装体分别包含 180 个和 360 个亚基。值得注意的是，这些膨胀颗粒呈现出前所未有的四面体和二十面体对称结构，完全由五聚体单元构成。壳中的大锁孔形孔（野生型衣壳中不存在）能够对带互补电荷的客体进行扩散限制的封装。这些增压组件的结构展示了如何有效地利用编程静电效应来定制蛋白质笼的结构和特性。