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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).

References associated with 1.K.1 family:

Aksyuk, A.A., P.G. Leiman, L.P. Kurochkina, M.M. Shneider, V.A. Kostyuchenko, V.V. Mesyanzhinov, and M.G. Rossmann. (2009). The tail sheath structure of bacteriophage T4: a molecular machine for infecting bacteria. EMBO. J. 28: 821-829. 19229296
Hu, B., W. Margolin, I.J. Molineux, and J. Liu. (2015). Structural remodeling of bacteriophage T4 and host membranes during infection initiation. Proc. Natl. Acad. Sci. USA 112: E4919-4928. 26283379
Leiman, P.G., F. Arisaka, M.J. van Raaij, V.A. Kostyuchenko, A.A. Aksyuk, S. Kanamaru, and M.G. Rossmann. (2010). Morphogenesis of the T4 tail and tail fibers. Virol J 7: 355. 21129200
Pukatzki, S., A.T. Ma, A.T. Revel, D. Sturtevant, and J.J. Mekalanos. (2007). Type VI secretion system translocates a phage tail spike-like protein into target cells where it cross-links actin. Proc. Natl. Acad. Sci. USA 104: 15508-15513. 17873062
Rossmann, M.G., V.V. Mesyanzhinov, F. Arisaka, and P.G. Leiman. (2004). The bacteriophage T4 DNA injection machine. Curr. Opin. Struct. Biol. 14: 171-180. 15093831
Shneider, M.M., S.A. Buth, B.T. Ho, M. Basler, J.J. Mekalanos, and P.G. Leiman. (2013). PAAR-repeat proteins sharpen and diversify the type VI secretion system spike. Nature 500: 350-353. 23925114