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).
Phage P2 portal protein of 344 aas (Rishovd et al. 1994).
PPP of Escherichia phage P2 (Bacteriophage P2)
Putative phage portal protein of 354 aas
PPP of Campylobacter fetus
Phage portal protein of 406 aas
PPP of Candidatus Lambdaproteobacteria bacterium (subsurface metagenome)
Putative phage portal protein of 481 aas.
PPP of Geobacillus thermodenitrificans
Phage portal protein of 560 aa
PPP of Nonlabens phage
Phage portal protein of 432 aa
PPP of Caldithrix abyssi
Streptomyces phage portal protein of 511 aas
PPP of Streptomyces phage Jay2Jay
Uncharacterized protein of 395 aas
UP of Candidatus Cloacimonas sp. SDB (marine sediment metagenome)
Phage portal protein of 523 aas
PPP of Streptomyces aureus
Uncharacterized protein of 374 aas
UP of Candidatus Woesearchaeota archaeon (marine sediment metagenome)
Uncharacterized protein of 405 aas
UP of Candidatus Methanoperedens nitroreducens
Phage portal protein of 648 aa
PPP of Bacillus phage 0305phi8-36
Uncharacterized protein DRN04_13085 of 461 aas
UP of Thermoprotei archaeon
Uncharacterized protein of 528 aas
UP of Candidatus Hydrothermarchaeota archaeon (marine sediment metagenome)
Phage portal protein of 526 aas
PPP of Desulfosarcina alkanivorans