Together with P22, forms the internal part of the portal complex embeded in the virion internal membrane 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 and interact with the P6/P9 external part of the portal. Heterodimer of P20 and P22; 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. Part of the capsid inner membrane. Binds the portal complex at the capsid vertex.