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

We have used cryoelectron microscopy and image reconstruction to study B-ca psids recovered from both the nuclear and the cytoplasmic fractions of cell s infected with simian cytomegalovirus (SCMV). SCMV, a representative betah erpesvirus, could thus be compared with the previously described B-capsids of the alphaherpes-viruses, herpes simplex virus type 1 (HSV-1) and equine herpesvirus 1 (EHV-1), and of channel catfish virus, an evolutionarily remo te herpesvirus. Nuclear B-capsid architecture is generally conserved with S CMV, but it is 4% larger in inner radius than HSV-1, implying that its simi lar to 30% larger genome should be packed more tightly. Isolated SCMV B-cap sids retain a relatively well preserved inner shell (or "small core") of sc affolding-assembly protein, whose radial-density profile indicates that thi s protein is similar to 16-nm long and consists of two domains connected by a low density linker. As with HSV-1, the herons but not the pentons of the major capsid protein (151 kDa) bind the smallest capsid protein (similar t o 8 kDa). Sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed cytoplasmic B-capsid preparations to contain proteins similar in molecular weight to the basic phosphoprotein (similar to 119 kDa) and the matrix prot eins (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 acc ordingly as lightly, moderately, or heavily tegumented. By comparing the th ree corresponding density maps with each other and with the nuclear B-capsi d, two interactions were identified between putative tegument proteins and the capsid surface. One is between the major capsid protein and a protein e stimated by electron microscopy to be 50 to 60 kDa; the other involves an e longated molecule estimated to be 100 to 120 kDa that is anchored on the tr iplexes, most likely on its dimer subunits. Candidates for the proteins bou nd at these sites are discussed. This first visualization of such linkages makes a step towards understanding the organization and functional rational e of the herpesvirus tegument.