The three-dimensional structure of a DNA translocating machine at 10 angstrom resolution

Background: Head-tail connectors are viral substructures that are very impo rtant in the viral morphogenetic cycle, having roles in the formation of th e precursor capsid (prohead), DNA packaging, tail binding to the mature hea d and in the infection process. Structural information on the connector wou ld, therefore, help us to understand how this structure is related to a mul tiplicity of functions. Results: Recombinant bacteriophage phi 29 connectors have been crystallized in two-dimensional aggregates. An average projection image and a three-dim ensional map have been obtained at 8 Angstrom and 10 Angstrom resolution, r espectively, from untilted and tilted images of vitrified specimens of the two-dimensional crystals. The average projection image reveals a central ma ss surrounding a channel with 12 appendages protruding from the central mas s. The three-dimensional map reveals a wide domain surrounded by 12 appenda ges that interact with the prohead vertex, and a narrow domain that interac ts with the bacteriophage tail, At the junction of the two domains, 12 smal ler appendages are visualized. A channel runs along the axis of the connect or structure and is sufficiently wide to allow a double-stranded DNA molecu le to pass through. Conclusions: The propeller-like structure of the phi 29 connector strengthe ns the notion of the connector rotating during DNA packaging. The groove fo rmed by the two lanes of large and small appendages may act as a rail to pr event the liberation of the connector from the prohead vertex during rotati on.