Scaffolding protein involved in the icosahedric procapsid assembly. Coassembles with the capsid proteins to form the procapsid, in which the scaffolding protein is found within the external shell of icosahedrally arranged capsid protein subunits. In a subsequent step the scaffolding protein molecules are cleaved by the viral protease and released, except for the internal peptide VII. Cleavage product of Gp22 that is incorporated into the mature phage head. Belongs to the T4likevirus capsid assembly scaffolding protein family.