The structure of bacteriophage T4 gene product 9: the trigger for tail contraction

Background: The T4 bacteriophage consists of a bead, filled with double-str anded DNA, and a complex contractile tail required for the ejection of the viral genome into the Escherichia coli host. The tail has a baseplate to wh ich are attached six long and six short tail fibers. These fibers are the s ensing devices for recognizing the host. When activated by attachment to ce ll receptors, the fibers cause a conformational transition in the baseplate and subsequently in the tail sheath, which initiates DNA ejection. The bas eplate is a multisubunit complex of proteins encoded by 15 genes. Gene prod uct 9 (gp9) is the protein that connects the long tail fibers to the basepl ate and triggers the tail contraction after virus attachment to a host cell . Results: The crystal structure of recombinant gp9, determined to 2.3 Angstr om resolution, shows that the protein of 288 amino acid residues assembles as a homotrimer. The monomer consists of three domains: the N-terminal doma in generates a triple coiled coil; the middle domain is a mixed, seven-stra nded beta sandwich with a topology not previously observed; and the C-termi nal domain is an eight-stranded, antiparallel beta sandwich having some res emblance to 'jelly-roll' viral capsid protein structures. Conclusions: The biologically active form of gp9 is a trimer. The protein c ontains flexible interdomain hinges, which are presumably required to facil itate signal transmission between the long tail fibers and the baseplate. S tructural and genetic analyses show that the C-terminal domain is bound to the baseplate, and the N-terminal coiled-coil domain is associated with the long tail fibers.