Capsid expansion of bacteriophage T5 revealed by high resolution cryoelectron microscopy

Significance Tailed DNA bacteriophages are the most ubiquitous viruses on earth. They are highly diverse but related to the medically important herpesviruses. We studied the protein shell, or capsid, that contains and protects the viral genome. One bacteriophage, called T5, has a large capsid made using about 800 copies of a protein organized with icosahedral geometry. Using state-of-the-art cryo-EM, we determined the structure of the T5 capsid in molecular detail as well as intermediates in its formation. This sequence of capsid structures reveals dramatic changes that take place during the crucial capsid assembly step in the bacteriophage T5 life cycle and allow us to observe essential widely conserved steps of virus assembly on a scale never observed before.