The structure of isometric capsids of bacteriophage T4

three-dimensional structure of DNA-filled, bacteriophage T4 isometric capsi ds has been determined by means of cryoelectron microscopy and image recons truction techniques. The packing geometry of protein subunits on the capsid surface was confirmed to be that of the triangulation class T = 13. The re construction clearly shows pentamers, attributed to capsid protein gp24*, s urrounded by hexamers of the major capsid protein, gp23*. Positions of the accessory proteins, Hoc and Sec, 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 g lobular head protrude. One Hoc molecule associates with each hexamer. Nearl y continuous "ridges" are formed at the periphery of the gp23* hexamers by an association of 12 Soc molecules; however, Soc is absent along the bounda ries between the hexamers and the pentamers. The duplex DNA genome forms a highly condensed series of concentric layers, spaced about 2.36 nm apart, t hat follow the general contour of the inner wall of the protein capsid. (C) 2001 Academic Press.