Fig. 7. Conformational plasticity of the triplex complex.

a In the extended conformation, neighboring triplex complexes make handshaking contacts with gp44-a pin domains projecting toward the tail tip. b Gp44-a pin domain is ~70° away from the C-terminal domain. c In the contracted conformation, neighboring triplex complexes make finger-tip contacts, and d gp44-a pin domains swing away from the tail tip, collapsing onto their C-terminal domains. e Section views of the baseplate before (left) and after (right) sheath contraction. Pin domain retraction causes lumen expansion and tail tip release.