CAPSID β-HINGE OF P22. “This hinge appears to direct motion of the A-domain during hexon symmetrization that occurs simultaneously with expansion. Finally, plug proteins (gp4, gp10 and gp26) close the portal protein channel and tailspike proteins (gp9) are added to complete the mature infectious phage.”
http://www.sciencedirect.com/science/article/pii/S0042682215001920