Tailed bacteriophages and herpesviruses build their capsids by co-assembling their major capsid protein with an internal scaffolding protein that is removed from the prohead intact or in pieces after it is digested by a maturation protease. Phage HK97's scaffolding protein is the 102 residue N-terminal delta domain of the major capsid protein which is also removed by proteolysis after assembly by a protease, which we have characterized. The protease has N-terminal cleavage and C-terminal packaging domains.