Major capsid protein that self-associates to form 120 hexamers and 11 pentamers, building the T=13 icosahedral capsid which about 860 Angstroms in diameter. Responsible for its self-assembly into a procapsid. The phage does not need to encode a separate scaffolfing protein because its capsid protein contains the delta domain that carries that function. The capsid gains its final stability through the reorganization of the subunits that takes place upon expansion. DNA encapsidation through the portal triggers capsid expansion and the binding of the decoration protein to the capsid exterior. Might play a role in counteracting the host Pycsar defense system that is mediated by pyrimidine cyclases and leads to abortive infection. Interacts with the decoration protein; each hexon binds a single copy of the decoration protein. Interacts with the portal protein (Probable). Forms the icosahedral capsid shell which contains 775 major capsid proteins. The scaffolding domain delta has a role of scaffold allowing the self-assembly of the capsid protein. The scaffolding domain delta is cleaved by the prohead protease and lost after assembly (PubMed:16876823). The major capsid protein precursors together with both the portal complex and the maturation protease form prohead I (PubMed:26616586). All copies of the major capsid protein precursor are cleaved to the mature major capsid protein by release of the scaffolding domain delta, yielding the metastable prohead II (PubMed:26616586). Belongs to the HK97 phage major capsid protein family. Truncated N-terminus.