Interaction of frog virus 3 with the cytomatrix. II. Structure and composition of the virus assembly site.

We have described the structure of virus assembly sites in frog virus 3-infected tissue culture cells based on an examination of sectioned and whole cells by conventional and high voltage (1 000 kV) electron microscopy (HVEM), respectively. We have also attempted to identify the cellular and viral components within the assembly sites using immunofluorescence and a combination of DNase digestion and EM autoradiography. Immunofluorescence studies showed that the sites do not contain tubulin, vimentin, actin, or myosin, confirming EM and HVEM studies which showed the absence of these cytoskeletal filaments from these sites. Enzymatic digestion and autoradiographic studies identified viral DNA in clumps of electron-dense material which are suspended within the matrix of the assembly site. Examination of whole, virus-infected cells by HVEM revealed that the matrix of the assembly site appears as a compaction of the cytoplasmic matrix and appears to be continuous with it. The fine strands of cytomatrix, equated with the microtrabeculae of other cells, are covered with or contain granules measuring 6-8 nm; similar granules also populate the virus assembly sites. Disruption of purified FV 3 with chloroform yielded capsomeres measuring 6-8 nm. Based on all these observations, we postulate that the virus assembly sites are regions of the cytomatrix specialized for virus assembly and that the viral components are transported along the cytomatrix to the assembly sites.