Cryo-EM structures of an Acinetobacter baumannii podovirus reveal a canonical capsid maturation

Phages that hunting multidrug resistant bacteria represent a promising approach in precision medicine nowadays, including these infecting Acinetobacter baumannii. To data, a large number of A. baumannii phages has been isolated, however, complete structural information remains not adequately released. Previously, we reported a podovirus, AbP20, that infects A. baumannii with potent efficacy in mouse model. In the present work, we further report the high-resolution cryo-electron microscopy (cryo-EM) structures of AbP20 and its procapsid. The near-atomic resolution reveals that AbP20 is composed of five distinct structural subunits, i.e., capsid, portal, nozzle, fiber, and adaptor. We then characterized the assembly mechanisms of molecular interactions within capsid and portal-tail complex. By co-expressing the scaffold protein and the capsid protein of AbP20, we constructed the self-assembled DNA-free procapsid of AbP20. A near-atomic resolution reconstruction reveals it a double-layered concentric spherical architecture with a nearly 1:1 ratio of these two proteins. Comparison between the mature and apo-form of the capsid of AbP20 uncovers a canonical model of conformational switch from procapsid to capsid. Altogether, the present work reports the first whole atom structure of a podovirus infecting A. baumannii, providing a reference for structure-based phage engineering and editing.