Genome delivery of a contractile tailed phage and its superinfection exclusion mechanism

Successful viral infection requires efficient adsorption to the target cell, followed by membrane penetration for genome translocation into the host cytoplasm. Bacteriophage T4 initiates infection by recognising Escherichia coli surface receptors via its long tail fibres. Receptor binding triggers sequential conformational changes that culminate in tail sheath contraction and genome delivery through viral channels spanning the host cell envelope. Despite extensive structural studies, the mechanism of genome translocation by tailed phages remains unclear. Here, using cryo-electron microscopy, we resolved structures of bacteriophage T4 at discrete stages. This revealed how long tail fibre extension affects the baseplate to initiate tail contraction, and the domain architecture of the tape measure protein and its involvement in channel formation and genome translocation. Furthermore, we demonstrate that the virus-encoded superinfection exclusion protein Imm binds to the tape measure protein to prevent secondary infections. Our findings reveal the mechanism of tape measure protein-mediated membrane penetration, offering insights into the coordinated process of genome delivery and phage-encoded superinfection exclusion proteins that prevent genome translocation.