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

Head-tail connectors are viral substructures that are very important in the viral morphogenetic cycle, having roles in the formation of the precursor capsid (prohead), DNA packaging, tail binding to the mature head and in the infection process. Structural information on the connector would, therefore, help us to understand how this structure is related to a multiplicity of functions.

Recombinant bacteriophage phi29 connectors have been crystallized in two-dimensional aggregates. An average projection image and a three-dimensional map have been obtained at 8 A and 10 A resolution, respectively, from untilted and tilted images of vitrified specimens of the two-dimensional crystals. The average projection image reveals a central mass surrounding a channel with 12 appendages protruding from the central mass. The three-dimensional map reveals a wide domain surrounded by 12 appendages that interact with the prohead vertex, and a narrow domain that interacts with the bacteriophage tail. At the junction of the two domains, 12 smaller appendages are visualized. A channel runs along the axis of the connector structure and is sufficiently wide to allow a double-stranded DNA molecule to pass through.

The propeller-like structure of the phi29 connector strengthens the notion of the connector rotating during DNA packaging. The groove formed by the two lanes of large and small appendages may act as a rail to prevent the liberation of the connector from the prohead vertex during rotation.