The structurome of a Clostridium difficile phage and the remarkable accurate prediction of its novel phage receptor-binding protein

As natural bacterial predators, bacteriophages have the potential to be developed to tackle antimicrobial resistance, but our exploitation of them is limited by understanding their vast uncharacterised genetic diversity 1,2 . Fascinatingly, this genetic diversity reflects many ways that phages can make proteins, performing similar functions that together form the familiar phage particle. Critical to infection are phage receptor-binding proteins (RBPs) that bind bacterial ‘receptors’ and initiate bacterial entry 3 . Here we identified and characterised Gp22, a novel RBP for phage CDHS-1 that infects pathogenic C. difficile , but that had no recognisable RBPs. We showed that Gp22 antibodies neutralised CDHS-1 infection and used immunogold-labelling and transmission electron microscopy to identify their location on the capsid. The Gp22 three-dimensional structure was resolved by X-ray crystallography revealing a new RBP class with an N-terminal L-shaped α-helical superhelix domain and a C-terminal Mg 2+ -binding domain. The findings provide novel insights into C. difficile phage biology and phage-host interactions. This will facilitate optimal phage development and future engineering strategies 4,5 . Furthermore, the AlphaFold2-predicted Gp22 structure, which was strikingly accurate, paves the way for a structurome based transformation and guidance of future phage studies where many proteins lack sequence homology but have recognisable protein structures.