Crystal structure of pb9, the distal tail protein of bacteriophage T5: a conserved structural motif among all siphophages.

The tail of Caudovirales bacteriophages serves as an adsorption device, a host cell wall-perforating machine, and a genome delivery pathway. In Siphoviridae, the assembly of the long and flexible tail is a highly cooperative and regulated process that is initiated from the proteins forming the distal tail tip complex. In Gram-positive-bacterium-infecting siphophages, the distal tail (Dit) protein has been structurally characterized and is proposed to represent a baseplate hub docking structure. It is organized as a hexameric ring that connects the tail tube and the adsorption device. In this study, we report the characterization of pb9, a tail tip protein of Escherichia coli bacteriophage T5. By immunolocalization, we show that pb9 is located in the upper part of the cone of the T5 tail tip, at the end of the tail tube. The crystal structure of pb9 reveals a two-domain protein. Domain A exhibits remarkable structural similarity with the N-terminal domain of known Dit proteins, while domain B adopts an oligosaccharide/oligonucleotide-binding fold (OB-fold) that is not shared by these proteins. We thus propose that pb9 is the Dit protein of T5, making it the first Dit protein described for a Gram-negative-bacterium-infecting siphophage. Multiple sequence alignments suggest that pb9 is a paradigm for a large family of Dit proteins of siphophages infecting mostly Gram-negative hosts. The modular structure of the Dit protein maintains the basic building block that would be conserved among all siphophages, combining it with a more divergent domain that might serve specific host adhesion properties.