Major capsid protein that self-associates to form hexamers, building most of the capsid in association with pentons made of the capsid vertex protein and one dodecamer of the portal protein. Homohexamer. Interacts with the portal protein. Interacts with the capsid vertex protein that forms pentamers. Part of the capsid icosahedric shell of the immature virion. Part of the icosahedric capsid shell of the mature virion. A proteolytic cleavage gives rise to the mature major capsid protein during virus maturation. Belongs to the T4 phage capsid protein family.