Self-assembles to form an icosahedral capsid with a T=7 symmetry. Interacts (via N-terminus) with the capsid assembly scaffolding protein (via C-terminus); capsid proteins and scaffolding proteins form building blocks that assemble to form the procapsid. Forms the capsid icosahedric shell. The insertion domain (I domain) plays an important role in the folding of the capsid protein by acting as an intramolecular chaperone. Belongs to the P22 phage major capsid protein family.