The number of different virion structural proteins ranges from 7-49. Typical head shells
are made up of 60T molecules of a single main building block CP and 12 molecules of
portal protein through which DNA enters and leaves, but they can also contain varied
numbers of proteins that plug the portal hole, proteins to which tails bind, proteins that
bind to the outside of the CP shell (decoration proteins) and other proteins whose roles
are not known. Non-contractile tails are made of one major shaft or tube protein and
contractile tails have a second major protein, the sheath protein, that forms a cylinder
around the central tube. Tails also have small numbers of varied specific proteins at both
ends. Those at the end distal from the head form a structure called the tail tip
(Siphovirus) or baseplate (Myovirus) to which the tail fibers are attached. The tail fibers
bind to the first-contact receptors on the surface of susceptible cells. Fibers or baseplates
may include proteins with endoglycosidase or peptidoglycan hydrolase activity that aid in
gaining access to the cell surface and entry of DNA into the cell. Most virions carry
proteins that are injected with the DNA, such as transcription factors, RNA polymerase
and others with poorly understood functions.