12-Fold symmetry of the putative portal protein from the Thermus thermophilus bacteriophage G20C determined by X-ray analysis.

In tailed bacteriophages and several animal viruses, the portal protein forms the gateway through which viral DNA is translocated into the head structure during viral particle assembly. In the mature virion the portal protein exists as a dodecamer, while recombinant portal proteins from several phages, including SPP1 and CNPH82, have been shown to form 13-subunit assemblies. A putative portal protein from the thermostable bacteriophage G20C has been cloned, overexpressed and purified. Crystals of the protein diffracted to 2.1 Å resolution and belonged to space group P42(1)2, with unit-cell parameters a = b = 155.3, c = 115.4 Å. The unit-cell content and self-rotation function calculations indicate that the protein forms a circular 12-subunit assembly.