Crystal structure of bacteriophage SPP1 distal tail protein (gp19.1): a baseplate hub paradigm in gram-positive infecting phages.

Siphophage SPP1 infects the gram-positive bacterium Bacillus subtilis using its long non-contractile tail and tail-tip. Electron microscopy (EM) previously allowed a low resolution assignment of most orf products belonging to these regions. We report here the structure of the SPP1 distal tail protein (Dit, gp19.1). The combination of x-ray crystallography, EM, and light scattering established that Dit is a back-to-back dimer of hexamers. However, Dit fitting in the virion EM maps was only possible with a hexamer located between the tail-tube and the tail-tip. Structure comparison revealed high similarity between Dit and a central component of lactophage baseplates. Sequence similarity search expanded its relatedness to several phage proteins, suggesting that Dit is a docking platform for the tail adsorption apparatus in Siphoviridae infecting gram-positive bacteria and that its architecture is a paradigm for these hub proteins. Dit structural similarity extends also to non-contractile and contractile phage tail proteins (gpV(N) and XkdM) as well as to components of the bacterial type 6 secretion system, supporting an evolutionary connection between all these devices.