Virus DNA translocation: progress towards a first ascent of mount pretty difficult.

Virion DNA molecules of large dsDNA viruses are highly condensed. To pack the DNA, an ATP hydrolysis-powered motor translocates the DNA into a preformed empty protein shell, the prohead. The icosahedral prohead has a special fivefold vertex, the portal vertex, where the translocation machinery acts. The portal vertex contains the portal protein, a gear-shaped dodecamer of radially disposed subunits with a central channel for DNA entry. The symmetry mismatch between the fivefold symmetry of the shell vertex and the 12-fold symmetry of the portal protein has prompted DNA packaging models in which ATP-driven portal protein rotation drives DNA translocation. In this issue of Molecular Microbiology, Baumann and colleagues test portal rotation models using bacteriophage T4. A fusion between the gp20 portal protein and the HOC external shell decoration protein is used to create a block to portal rotation. Finding that DNA packaging is unimpeded in proheads containing the fusion argues that portal rotation is not crucial to DNA translocation. The paper is a landmark for describing direct testing of the mechanism of DNA translocation.