For a three-dimensional structure to spontaneously self-assemble from many identical components, the steps on the pathway must be kinetically accessible. Many virus capsids are icosahedral and assembled from hundreds of identical proteins, but how they navigate the assembly process is poorly understood. Capsid assembly is thought to involve stepwise addition of subunits to a growing capsid fragment. Coarse-grained models suggest that the reaction occurs on a downhill energy landscape, so intermediates are expected to be fleeting. In this work, charge detection mass spectrometry (CDMS) has been used to track assembly of the hepatitis B virus (HBV) capsid in real time. The icosahedral T = 4 capsid of HBV is assembled from 120 capsid protein dimers. Our results indicate that there are multiple pathways for assembly. Under conditions that favor a modest association energy there is no accumulation of large intermediates, which indicates that available pathways include ones on a downhill energy surface. Under higher salt conditions, where subunit interactions are strengthened, around half of the products of the initial assembly reaction have masses close to the T = 4 capsid and the other half are stalled intermediates which emerge abruptly at around 90 dimers, indicating a bifurcation in the ensemble of assembly paths. When incubated at room temperature, the 90-dimer intermediates accumulate dimers and gradually shift to higher mass and merge with the capsid peak. Though free subunits are present in solution, the stalled intermediates indicate the presence of a local minima on the energy landscape. Some intermediates may result from hole closure, where the growing capsid distorts to close the hole due to the missing capsid proteins or from a species where subsequent additions are particularly labile.

中文翻译：

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衣壳组装中的多种途径

对于从许多相同的组件自发自组装的三维结构，路径上的步骤必须是动力学可访问的。许多病毒衣壳是二十面体的，由数百种相同的蛋白质组装而成，但它们如何在组装过程中导航却知之甚少。衣壳组装被认为涉及向生长的衣壳片段逐步添加亚基。粗粒度模型表明反应发生在下坡的能源景观中，因此中间体预计会转瞬即逝。在这项工作中，电荷检测质谱 (CDMS) 已被用于实时跟踪乙型肝炎病毒 (HBV) 衣壳的组装。HBV 的二十面体 T = 4 衣壳由 120 个衣壳蛋白二聚体组装而成。我们的结果表明有多种组装途径。在有利于适度缔合能量的条件下，没有大型中间体的积累，这表明可用途径包括下坡能量表面上的途径。在高盐条件下，亚基相互作用得到加强，初始组装反应的大约一半产物的质量接近 T = 4 衣壳，另一半是停滞的中间体，在大约 90 个二聚体处突然出现，表明在装配路径的集合。在室温下孵育时，90 二聚体中间体积累二聚体并逐渐转移到更高质量并与衣壳峰合并。尽管溶液中存在自由亚基，但停滞的中间体表明能量景观中存在局部最小值。一些中间体可能是由于孔闭合造成的，