Phage tail terminator protein (TrP) plays an essential role in phage tail assembly by capping the rapidly polymerizing tail once it has reached its requisite length and serving as the interaction surface for phage heads. Structural domains comprising this superfamily share the structure of the phage minor tail protein U, also designated as gpU, the TrP of phage lambda. gpU forms a hexameric ring within the tail structure since it spontaneously forms such rings in the presence of Mg2+ that match the size of the hexameric rings of gpV that comprise the bulk of the tail tube. However, prior to tail assembly, gpU remains monomeric PMID:2150582.