Abstract text (incl. figure legends and references)
Tailed bacteriophages use copies of a major capsid protein (MCP) with a common fold to build capsids capable to harbour dsDNA genomes spanning from tens to hundreds of kilobase pairs. Larger capsids are usually constructed by a higher number of MCPs, resulting in a larger triangulation number or in oblongate capsids. However, the continuous evolutionary pressure imposes phage capsids to adapt to various packaging load sizes. For instance, a relatively modest increase in the genome size cannot justify the transition to a novel capsid model, constructed from more MCPs. Instead it calls for strategies that increase the size of the capsid, while maintaining the same triangulation number.
Using cryo-electron microscopy (cryoEM), we solved the architecture of the phiTE phage in several intermediate conformations. The full capsid resolved at 3.4 Å helped us to decipher the structural mechanism by which phiTE capsid is able to accommodate a genome up to 40% larger than that of phages with the same capsid geometry (T=13). The strategy combines three factors: the increase of capsomer surface without an increase in MCP molecular weight, the relative sliding between individual capsomers, and the increase of local capsomer curvature.
Additionally, our cryoEM analysis offers a series of important insights into understanding the general principles of phage architecture. First, we describe two decoration proteins with distinct roles in phage stability and demonstrate that for decoration proteins belonging to the same class, a structure base classification does not mirror the classification based on amino acid alignment. Secondly, we follow the MCP conformations found in the spent empty capsid that displays an expanded geometry. Thirdly, we describe the components of the phiTE tail in the extended and contracted conformations. Finally, we show how the phiTE base plate employs a novel type of conformational change during genome ejection dictated by the reorientation of the tail fibres.