Mycobacteriophage Bxb1 Cryo-EM Composite Maps and Models

Mycobacteriophage Bxb1 is a well-characterized virus of Mycobacterium smegmatis with double-stranded DNA and a long flexible tail. Mycobacteriophages show considerable potential as therapies for Mycobacterium infections, but little is known of the structural details of these phages or how they bind to and traverse the complex Mycobacterium cell wall. Here we report the complete structure and atomic model of phage Bxb1, including the arrangement of immunodominant domains of both the capsid and tail tube subunits, and the assembly of the protein subunits in the tail tip complex. The structure contains protein assemblies with three-fold, five-fold, six-fold, and 12-fold symmetries, which interact to satisfy several symmetry mismatches. Cryo-electron tomography of phage particles bound to M. smegmatis reveals the structural transitions that occur for free phage particles to bind to the cell surface and navigate through the cell wall to enable DNA injection into the cytoplasm.

分枝杆菌噬菌体Bxb1（Mycobacteriophage Bxb1）是一类特征明确的耻垢分枝杆菌（Mycobacterium smegmatis）病毒，其遗传物质为双链DNA（double-stranded DNA）并带有一条长柔性尾。分枝杆菌噬菌体在治疗分枝杆菌感染领域展现出巨大应用潜力，但目前学界对这类噬菌体的结构细节，以及它们如何结合并穿越复杂的分枝杆菌细胞壁的分子机制仍知之甚少。本研究报道了噬菌体Bxb1的完整结构与原子模型，涵盖衣壳（capsid）与尾管亚基（tail tube subunits）的免疫显性结构域排布，以及尾尖复合物（tail tip complex）内的蛋白质亚基组装模式。该结构包含具有三重、五重、六重及十二重对称性的蛋白质组装体，这些组装体相互协作以解决多种对称性错配问题。对结合至耻垢分枝杆菌的噬菌体颗粒开展的冷冻电子断层扫描（Cryo-electron tomography）实验，揭示了游离噬菌体颗粒结合细胞表面、穿越细胞壁以将DNA注入细胞质（cytoplasm）过程中发生的结构动态转变。