Capsid structure of simian cytomegalovirus from cryoelectron microscopy: evidence for tegument attachment sites

We have used cryoelectron microscopy and image reconstruction to study B-capsids recovered from both the nuclear and the cytoplasmic fractions of cells infected with simian cytomegalovirus (SCMV). SCMV, a representative betaherpesvirus, could thus be compared with the previously described B-capsids of the alphaherpesviruses, herpes simplex virus type 1 (HSV-1) and equine herpesvirus 1 (EHV-1), and of channel catfish virus, an evolutionarily remote herpesvirus. Nuclear B-capsid architecture is generally conserved with SCMV, but it is 4% larger in inner radius than HSV-1, implying that its approximately 30% larger genome should be packed more tightly. Isolated SCMV B-capsids retain a relatively well preserved inner shell (or "small core") of scaffolding-assembly protein, whose radial-density profile indicates that this protein is approximately 16-nm long and consists of two domains connected by a low-density linker. As with HSV-1, the hexons but not the pentons of the major capsid protein (151 kDa) bind the smallest capsid protein (approximately 8 kDa). Sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed cytoplasmic B-capsid preparations to contain proteins similar in molecular weight to the basic phosphoprotein (approximately 119 kDa) and the matrix proteins (65 to 70 kDa). Micrographs revealed that these particles had variable amounts of surface-adherent material not present on nuclear B-capsids that we take to be tegument proteins. Cytoplasmic B-capsids were classified accordingly as lightly, moderately, or heavily tegumented. By comparing the three corresponding density maps with each other and with the nuclear B-capsid, two interactions were identified between putative tegument proteins and the capsid surface. One is between the major capsid protein and a protein estimated by electron microscopy to be 50 to 60 kDa; the other involves an elongated molecule estimated to be 100 to 120 kDa that is anchored on the triplexes, most likely on its dimer subunits. Candidates for the proteins bound at these sites are discussed. This first visualization of such linkages makes a step towards understanding the organization and functional rationale of the herpesvirus tegument.