Assembles to form an icosahedral capsid. The assembly is primed by the interaction between capsid assembly protease and portal dodecamer, and major capsid proteins assemble cooperatively to form the procapsid with the help of capsid scaffolding protein. Major capsid protein forms hexons and pentons of the icosahedron. Viral genomic DNA is packaged into the procapsid through the portal vertex. The packaging triggers a dramatic reconfiguration of the capsid shell. Homomultimer. Forms the capsid icosahedric shell. Belongs to the lambda phage major capsid protein family.