Stabilizes the expansion of the capsid head shell after genome packaging. The packaging of viral genome in the procapsid triggers a dramatic reconfiguration of the capsid shell, expanding from roughly 50nm to 60nm while the capsid thickness decreases. 415 capsid decoration protein molecules cooperatively bind the expanded capsid, thereby stabilizing the mature capsid shell. Homotrimer. Interacts with major capsid protein. Belongs to the lambda phage gpD family.