1.K.1 The Gp27/5 T4-baseplate (T4-BP) Family
The contractile tail of bacteriophage T4 is a molecular machine that facilitates very high viral infection efficiency. Its major component is a tail sheath, which contracts during infection to less than half of its initial length. The sheath consists of 138 copies of the tail sheath protein, gene product (gp) 18, which surrounds the central non-contractile tail tube. Contraction of the sheath drives the tail tube through the outer membrane, creating a channel for viral genome delivery. A crystal structure of about three quarters of gp18 has been determined and was fitted into cryo-electron microscopy reconstructions of the tail sheath before and after contraction (Aksyuk et al. 2009). It was shown that during contraction, gp18 subunits slide over each other with no apparent change in their structure. Gp27/Gp5 make up the baseplate ''''tail spike'''' which penetrates the membrane of E. coli and allows passage of DNA into the cell (Pukatzki et al. 2007). The tail complex of T4 resembles the type VI protein secretion systems (TC# 3.A.23) of enteric bacteria (Shneider et al. 2013).
Tail contraction is triggered by structural changes in the baseplate, as suggested by the observation that intermediates have remodeled baseplates and extended tails. After contraction, the tail tube penetrates the host cell periplasm and pauses while it degrades the peptidoglycan layer. Penetration into the host cytoplasm is accompanied by a dramatic local outward curvature of the cytoplasmic membrane as it fuses with the phage tail tip. The baseplate hub protein gp27 and/or the ejected tape measure protein gp29 may form the transmembrane channel for viral DNA passage into the cell cytoplasm (Hu et al. 2015).