Together with the packaging ATPase P9, forms the external part of the portal vertex that is embeded in the capsid and which plays critical roles in genome packaging and genome ejection. Both proteins multimerize as a single ring-shaped heterdodecamer arranged around a central channel. Heterodimer of P6 and P9; further multimerizes as hexamers of heterodimers. Part of the dodecameric portal complex that is composed of the packaging efficiency factor P6, the DNA packaging ATPase P9, and the internal heterododecamer P20/P22 which spans the virion inner membrane. Localizes at the unique vertex of the capsid, which extends to the internal membrane via two small integral membrane proteins, P20 and P22. Packaging efficiency factor P6 attaches to the virion via these proteins and is, in turn, necessary for the attachment of DNA packaging protein P9 to the capsid.