About bacteriophage tail terminator and tail completion proteins: structure of the proximal extremity of siphophage T5 tail

Abstract

Bacteriophages are viruses infecting bacteria. The vast majority of them bear a tail, allowing host recognition, cell wall perforation and DNA injection into the host cytoplasm. Using electron cryo-microscopy (cryo-EM) and single particle analysis, we determined the organisation of the tail proximal extremity of siphophage T5 that possess a long flexible tail, and solved the structure of its tail terminator protein (TrP) p142 (TrP142). It allowed to confirm the common evolutionary origin between T5 TrPp142 and other known or putative TrPs from siphophages, myophages and bacterial tail-like machines, despite very poor sequence conservation. By also determining the structure of T5 tail proximal extremity after interaction with T5 bacterial receptor FhuA, we showed that no conformational changes occur in TrPp142 and confirmed that the infection signal transduction is not carried by the tube itself. We also investigated the location of T5 tail completion protein (TCP) p143 (TCPp143) and showed, thanks to a combination of cryo-EM and structure prediction using Alphafold2, that it is not located at the capsid-to-tail interface as suggested by its position in the genome, but instead, very unexpectedly, on the side of T5 tail tip, and that it appears to be monomeric. Based on structure comparison with other putative TCPs predicted structures, this feature could not be shared by other TCPs. The stoichiometry of the Tape Measure Protein is also discussed. ImportanceBacteriophages, viruses infecting bacteria, are the most abundant living entities on Earth. They are present in all ecosystems where bacteria develop and are instrumental in the regulation, diversity, evolution and pathogeny of microbial populations. Moreover, with the increasing number of pathogenic strains resistant to antibiotics, virulent phages are considered as a serious alternative or complement to classical treatments. 96% of all phages present a tail that allows host recognition and safe channelling of the DNA to the host cytoplasm. We present the atomic model of the proximal extremity of siphophage T5 tail, confirming structural similarities with other phages. This structure, combined to results previously published further explored, also allowed a review and a discussion on the role and localisation of a mysterious tail protein, the Tail Completion Protein, which is known to be present in the phage tails, but that was never identified in a phage structure.