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Phage T4 cell-puncturing device

In order to be replicated phage DNA has to be transferred from the phage head to the host cell. As hosts won't like this particularly, the phages had to invent some efficient machinery to overcome the host's defence line. Coliphage T4 got an ingenious cellwall-puncturing device at the lower end of it's tail forming a part of the baseplate. The device is made up of two gene products (gp27 and gp5). The original gp5 is cleaved in vivo into two proteins, but the whole device (being of threefold symmetry) works as an intimately stuck nonameric complex.

gp27 mediates between the baseplate (which has sixfold symmetry) and the trimeric gp5 complex. One gp27 monomer has a large beta sheet which is wrapped into a distorted barrel which in the trimer forms a collar prolonging the phage's tail tube. The other part of gp27 binds gp5  .

The structure of gp5 is made up of three domains. The aminoterminal domain with it's fivestranded beta barrel contains the oligonucleotide/oligosaccharide binding fold. The binding side of this OB-fold however is completely surrounded in the whole complex by other subunits. It's potential binding capacity may be of use at some stage of the infection process after rearrangement of baseplate components.
The N-terminal OB-fold domain is connected by a linker to the lysozyme domain. This domain is nearly identical to the soluble lysozyme coded by T4 which is used for cell lysis after phage progeny synthesis is complete. Especially the catalytic triad of amino acids is positioned alike. In the trimeric gp5 complex the peptide-binding site is occupied by amino acids from a neighboring subunit  , as is a part of the polysaccharide binding cleft by the central part of the complex  . Therefore no catalytic activity is found in this state.
The linker connecting the lysozyme domain to the rest of the protein has a gap in this model (aminoacids 346-361 are missing) due to unordered structure in this loop  . Within this region is the cleavage point generating gp5* (OB-fold domain and lysozyme domain) and gp5C (C-terminal domain).
The structure of gp5C is in it's lower part obiously improbable. Indeed, it is viable only in the trimeric complex where distances between the beta strands allow the formation of sheets. This is a very rigid structure that is able to penetrate the outer membrane of prey bacteria like a hypodermic needle. The beta-helix is formed like a slightly tapered prism  . After penetration of this structure through the cell wall the lysozyme domains may flip from their positions as found in the crystal and start working on the peptidoglycan layer of the bacteria.
The extensive beta-sheet interactions of this molecular needle are the driving force for trimerization of the whole komplex. So the monomers of gp27 wouldn't form a trimer on their own, but only in complex with gp5.

Literature:
S Kanamura et al, Structure of the cell-puncturing device of bacteriophage T4, Nature 415 (2002) 553-557
AG Murzin, OB(oligonucleotide/oligosaccharide binding)-fold: common structural and functional solution for non-homologous sequences, EMBO J. 12 (1993) 861-867