Formation and role of the portal of Staphylococcus aureus bacteriophage 80α

Bacteriophages play an important role in the pathogenicity of Staphylococcus aureus, an important human pathogen. Phages are involved in generalized and specialized transduction as well as a more specific process by which they mobilize elements known as phage-inducible chromosomal islands, of which S. aureus pathogenicity islands (SaPIs) are an important group. SaPIs are mobilized at high frequency through interactions with specific "helper" bacteriophages, such as 80α, leading to packaging of the SaPI genomes into virions made from structural proteins supplied by the helper. Among these structural proteins is the portal protein, which forms a ring-like portal at a fivefold vertex of the capsid, through which the DNA is packaged during virion assembly and ejected upon infection of the host. We previously showed that portal protein expressed in E. coli forms tridecameric rings, while portals found in virions are always dodecamers. To understand the role of the portal in capsid assembly, DNA packaging and ejection, we have here examined this phenomenon further. We show that portals assembled at lower temperature form unclosed rings that may represent portal assembly intermediates. By analyzing portal protein deletion mutants, we demonstrate the involvement of the different functional domains in phage assembly and protein incorporation.