The structure of isometric capsids of bacteriophage T4

The three-dimensional structure of DNA-filled, bacteriophage T4 isometric capsids has been determined by means of cryoelectron microscopy and image reconstruction techniques. The packing geometry of protein subunits on the capsid surface was confirmed to be that of the triangulation class T = 13. The reconstruction clearly shows pentamers, attributed to capsid protein gp24*, surrounded by hexamers of the major capsid protein, gp23*. Positions of the accessory proteins, Hoc and Soc, are also clearly delineated in the surface lattice. The Hoc protein is the most prominent surface feature and appears as an extended molecule with a rounded base from which a thin neck and a globular head protrude. One Hoc molecule associates with each hexamer. Nearly continuous "ridges" are formed at the periphery of the gp23* hexamers by an association of 12 Soc molecules; however, Soc is absent along the boundaries between the hexamers and the pentamers. The duplex DNA genome forms a highly condensed series of concentric layers, spaced about 2.36 nm apart, that follow the general contour of the inner wall of the protein capsid.