Piercing mechanism in the minimal contractile tail of bacteriophage P2.
Bacteriophage P2 is a long-standing model for contractile injection systems, yet high-resolution structures of the intact virion and the mechanism of baseplate activation have remained unresolved. Here, we combine high-resolution cryo-electron microscopy and in situ cryo-electron tomography to characterize P2, using its lytic mutant P2vir, in its pre- and post-contraction states and during host infection. We identify a previously undescribed fold in the head-to-tail adaptor, which mediates the transition from 12-fold portal symmetry to sixfold tail symmetry through interlocking {beta}-strand handshake interactions. Our structures further reveal a compact, threefold-symmetric baseplate established by a distinctive loop-helix module formed by the hub and tube initiator, which bioinformatic and phylogenetic analyses identify as a characteristic architectural feature of the Peduovirus lineage. We also propose a rope-like arrangement that anchors the tail fibre to the baseplate triplex component gpI and obtain evidence that the tape-measure protein adopts a C3 arrangement within the tail lumen, while computational predictions suggest a possible hexameric, pore-like organization after release. Comparison of pre- and post-contraction particles, supported by in situ imaging of infection, suggests that sheath contraction destabilizes the baseplate, promoting its disassembly. The inner tail tube appears to penetrate the host outer membrane but does not appear to reach the inner membrane, leading us to propose that the reorganized tape-measure protein forms a secondary conduit for genome translocation. These findings support a distinct model of contractile tail activation and expand the known architectural diversity of viral injection systems.