The portal structure has been implicated in several aspects of the bacteriophage life cycle, including capsid assembly initiation and DNA packaging. Here we present evidence that P2 gene Q codes for the P2 and P4 portal protein. First, microsequencing shows that capsid protein h6 is derived from gpQ, most probably by proteolytic cleavage. Second, antibodies against gpQ bind to the portal structure in disrupted P2 phage virions, as observed by electron microscopy. Third, gpQ partially purified from an overexpressing plasmid assembles into portal-like structures. We also show by microsequencing that capsid protein h7 is encoded by the P2 scaffold gene, O, and is probably derived from gpO by proteolytic cleavage. Previous work has demonstrated processing of the major capsid protein. Thus, all essential capsid proteins of P2 and P4 are proteolytically cleaved during the morphogenetic process.