The members of this family are similar to gene products 9 (gp9) and 10 (gp10) of bacteriophage T4. Both proteins are components of the viral baseplate [1]. Gp9 (Swiss:P10927) connects the long tail fibres of the virus to the baseplate and triggers tail contraction after viral attachment to a host cell. The protein is active as a trimer, with each monomer being composed of three domains. The N-terminal domain consists of an extended polypeptide chain and two alpha helices. The alpha1 helix from each of the three monomers in the trimer interacts with its counterparts to form a coiled-coil structure. The middle domain is a seven-stranded beta-sandwich that is thought to be a novel protein fold. The C-terminal domain is thought to be essential for gp9 trimerisation and is organised into an eight- stranded antiparallel beta-barrel, which was found to resemble the 'jelly roll' fold found in many viral capsid proteins. The long flexible region between the N-terminal and middle domains may be required for the function of gp9 to transmit signals from the long tail fibres [2]. Together with gp11, gp10 (Swiss:P10928) initiates the assembly of wedges that then go on to associate with a hub to form the viral baseplate [1].

1: Bacteriophage T4 genome. Miller ES, Kutter E, Mosig G, Arisaka F, Kunisawa T, Ruger W; Microbiol Mol Biol Rev 2003;67:86-156. PMID:12626685

2: The structure of bacteriophage T4 gene product 9: the trigger for tail contraction. Kostyuchenko VA, Navruzbekov GA, Kurochkina LP, Strelkov SV, Mesyanzhinov VV, Rossmann MG; Structure Fold Des 1999;7:1213-1222. PMID:10545330