Decoration protein that binds asymmetrically to the center of each capsid protein hexamer after capsid expansion. Stabilizes the capsid and protects from DNA release. Interacts with the major capsid protein; each hexon binds a single copy of the decoration protein. 120 copies of the decoration protein are localized on the exterior of the capsid. The N-terminus binds to the capsid proteins with high affinity and high cooperativity while the C-terminus displays an IG-like fold.