Component of the cylindrical core that assembles on the inner surface of the capsid during procapsid formation. Required for stabilization of the condensed DNA within the capsid; perhaps by plugging the hole through which the DNA enters. Plays a role in ejection of the bacteriophage DNA into the host cell at the initiation of infection. Functions as an exolysin that catalyzes the cleavage of the glycosidic bonds between N-acetylmuramic acid and N-acetylglucosamine residues in peptidoglycans. To phage P22 gp4.