Conformational changes of baseplate linked to tail contraction of S. aureus phage phi812

Phages with contractile tails employ elaborate mechanisms to penetrate bacterial cell walls and deliver their genomes into the host cytoplasm. Most tailed phages that have been structurally characterized to date infect Gram-negative bacteria, but those targeting Gram-positive bacteria, many of which are important human pathogens, are less well understood. Here, we show that the baseplate of phage phi812, which infects Gram-positive Staphylococcus aureus , is formed of a core, wedge modules, and baseplate arms carrying receptor-binding proteins type 1 and 2 and tripod complexes. The baseplate of phi812 with extended tail has threefold symmetry but after tail contraction it transforms to sixfold symmetry. In this process, the receptor-binding proteins re-orient to bind to the host cell. The conformational changes to the tripod complexes trigger the release of the central spike and weld proteins from the baseplate, which exposes peptidoglycan-degrading domains of the hub proteins. Changes in the positions of baseplate arms are transmitted through wedge modules to tail sheath initiator proteins. The ring formed by the tail sheath initiator proteins expands in diameter and triggers the contraction of the tail sheath. The tail sheath of phi812, shortens from 200 to 96 nm, which pushes the tail tube 10-30 nm into the cytoplasm, depending on the local thickness of the S. aureus cell wall. Our results demonstrate how tail and baseplate enable phi812 genome delivery into Gram-positive S. aureus .