1.W.3. The Phage Portal Protein 3 (PPP3) Family
Members of this family form the portal vertex of the capsid (Rishovd et al. 1994). This portal plays important roles in head assembly, genome packaging, neck/tail attachment, and genome ejection. The portal protein multimerizes as a single ring-shaped homododecamer arranged around a central channel. It also binds to the terminase subunits to form the packaging machine.
The portal structure has been implicated in several aspects of the bacteriophage life cycle, including capsid assembly initiation and DNA packaging. Rishovd et al. 1994 presented evidence that P2 gene Q codes for the P2 and P4 portal protein. First, microsequencing showed 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 assembled into portal-like structures. They also showed by microsequencing that capsid protein h7 is encoded by the P2 scaffold gene, O, and is probably derived from gpO by proteolytic cleavage. All essential capsid proteins of P2 and P4 are proteolytically cleaved during the morphogenetic process (Rishovd et al. 1994).