The Shigella siphophage Sf11 tail structure and host attachment mechanism

A paucity of reports is available describing the structures of Shigella phages, and these have focused to date on a few short, non-contractile podophages and one long contractile myophage. Here, we report the cryo-EM structure of a Shigella siphophage, where we can visualize the capsid and surface decoration proteins and many components of the tail, including the Tail Tube Protein (TTP), Distal Tail Protein (DTP), Baseplate Hub Proteins (BHUB1 & BHUB2), and the Tape Measure Protein (TMP). The tail is also decorated with six copies of a trimeric tailspike protein that is similar to Sf6-like and P22-like phages. We used mass spectrometry to confirm the identity of the proteins in the mature virion and present atomic models for the majority of these phage proteins. In addition, host range studies show clearly that these tailspike appendages have a homologous function to those in the Sf6-like and P22-like phages in recognizing the O-antigen on the host lipopolysaccharide (LPS). IMPORTANCE Few Shigella phages have been studied structurally to date. By characterizing phage Sf11, we see evidence for a tail adaptor domain that is used for decorating the siphophage tail tip with enzymatic, P22-like tailspike proteins. This is important for both understanding the evolutionary relationships among Shigella phages and also could be exploited as a type of protein scaffolding for creating designer phages for therapeutic and/or industrial purposes.