Cooperatively binds the expanded capsid, thereby stabilizing the mature capsid shell and allowing the large viral DNA to be packaged (PubMed:30737287). Trimers of capsid decoration proteins molecules are located at local and icosahedral threefold axes and stabilize the expanded capsid, which shows increased spacing between capsomers (PubMed:30737287). Homotrimer (PubMed:30737287). Interacts with the major capsid protein (PubMed:30737287). Located between the capsid hexons.