Serine protease involved in capsid assembly and maturation. Cleaves the major capsid protein, the decoration protein, the portal protein to yield mature procapsids competent for DNA packaging (Probable). Acts as a trigger for assembly of the capsid protein. Present in the prohead I. Cleaves itself autocatalytically to yield the mature form of the protease (Probable). Belongs to the HK97 prohead protease protein family.