Molecular mechanism of bacteriophage tail contraction- structure of an S-layer-penetrating bacteriophage

Abstract

Viruses that infect bacteria (bacteriophages or phages) attach to the host cell envelope, inject their genetic material into the host cytosol and either persist as prophage or hijack the host machinery to produce progeny virions. Attachment is mediated through phage receptor binding proteins that are specific for different host cell surface molecules. A subset of phage, the myoviruses, possess contractile tails, the outer sheath of which contracts upon receptor binding, driving an inner tail tube through the cell envelope and delivering the phage genome into the host cytosol. The molecular details of phage tail contraction and mode of cell envelope penetration have remained poorly understood and were completely unknown for any phage infecting bacteria enveloped by a proteinaceous S-layer. Here we reveal the extended and contracted atomic structures of an intact contractile-tail phage that binds to and penetrates the protective S-layer of the Gram positive human pathogen Clostridioides difficile. Surprisingly, we find no evidence of the intrinsic enzymatic domains that other phages exploit in cell wall penetration, suggesting that sufficient energy is released upon tail contraction to penetrate the S-layer and the thick cell wall without enzymatic activity. However, it is also notable that the tail sheath subunits move less than those studied in related contractile injection systems such as the model phage T4. Instead, the unusually long tail length and flexibility upon contraction likely contribute towards the required free energy release for envelope penetration. Our results show that the principles of phage contraction and infection as determined in the model system of T4 are not universal. We anticipate that our structures will form a strong foundation to engineer C. difficile phages as therapeutics, and highlight important adaptations made in order to infect S-layer containing pathogens.