Bacteriophage T7 provides a well defined system to study the assembly of a moderately complex virus. The mature virion consists of a 60 nm icosahedral capsid containing the linear 40 kb genome and an internal protein “core”, and a short tail. The capsid originates as a DNA-free prohead, whose assembly requires both the major capsid protein (gpl0, ∼ 420 copies) and the scaffolding protein (gp9, ∼180 copies). Native proheads incorporate an oligomeric ring of the connector/portal protein at one vertex and the inner core, but prohead-like particles assemble when gp10 and gp9 alone are expressed. In maturation -as with other dsDNA phages - the T7 prohead undergoes a major conformational change, in which the shell structure alters dramatically, and the scaffolding protein is expelled. We have used cryo-EM in combination with other techniques to study this transition and to investigate the interactions between scaffold and surface shell in the T7 prohead.
Highly purified preparations of proheads were generated from the co-expression of head and scaffolding proteins from the cloned gene in E. coli. Tail-less mature capsids as well as DNA-filled heads were prepared from infection of a T7 strain that completely lacks the tail genes.