The contractile tail of bacteriophage T4: A molecular machine for infecting bacteria.

摘要

Bacteriophage T4 tail is a complex molecular machine that facilitates high infection efficiency of the virus. It consists of an inner rigid tail tube, an outer contractile tail sheath and a baseplate with fibers. The tail sheath is composed of 138 monomers of gene product (gp) 18, arranged into a six-start helix. The baseplate is located at the distal end of the sheath and consists of ∼150 polypeptide chains from 16 different proteins. Binding of the phage to the host cell induces sequential rearrangement of the baseplate, leading to the contraction of the tail sheath. During contraction, the tail sheath diameter increases, whereas its length decreases from 925 to 420 A. In the current work an X-ray structure of about three quarters of the tail sheath protein was determined and fitted into the cryo-EM reconstructions of the T4 tail before and after infection. The results suggest that sheath contraction occurs by sliding of individual gp18 molecules over each other as rigid bodies. It was identified that conformational changes are propagated to the sheath though the baseplate protein gp6. The structure of a C-terminal half of gp6 was determined by X-ray crystallography and the whole of gp6 was segmented from the cryo-EM maps of the baseplate. Six gp6 dimers interdigitate, forming a ring which maintains the integrity of the baseplate during the conformational change. Although interactions between neighboring gp6 molecules remain the same during baseplate rearrangement, the hinge angle between the N- and C-terminal parts of gp6 changes by ∼15°.;It had been previously shown that tail sheaths of different phages have similar helical parameters, despite no detectable sequence homology. A structure of a portion of the tail sheath protein from bacteriophage PhiKZ was solved by X-ray crystallography. The tail sheath protein of PhiKZ shares less then 15% sequence homology with gp18 of T4. Nevertheless, a portion of the tail sheath protein, involved in forming the intersubunit contacts, was found to have a similar fold as T4 gp18, providing another example of structural conservation among bacteriophage proteins and suggesting a similar sheath contraction mechanism in different phages.