Structural remodeling of bacteriophage T4 and host membranes during infection initiation

Significance The bacteriophage T4 tail is a complex nanomachine that undergoes a succession of structural changes as it infects a bacterium. We analyzed cryo-electron microscopic images of T4 at different stages of infection. Three-dimensional visualization of key intermediates revealed unprecedented structural details, allowing a better understanding of this fundamental and highly efficient process. Contrary to common descriptions, most long tail fibers are folded back against the virion before infection, and not all interact with the cell before the short tail fibers irreversibly bind. Sheath contraction drives the tail tube only into the periplasm where, unexpectedly, the cytoplasmic membrane bulges outwards to fuse with the tail tube. Fusion does not require the proton motive force, which only becomes necessary for genome translocation.