Replication of Sindbis Virus IV. Electron Microscope Study of the Insertion of Viral Glycoproteins into the Surface of Infected Chick Cells

The appearance of Sindbis virus-envelope glycoproteins in the surfaces of chicken embryo fibroblasts was studied by an indirect labeling technique. This technique involved treating infected cells sequentially with rabbit immunoglobulin G (IgG) specific for Sindbis virus followed by hemocyanin-conjugated goat (anti-rabbit IgG) IgG; surface replicas of these cells were then prepared and examined in the electron microscope. As early as 2 h after infection (and at least 1 h before mature virions were released), newly synthesized virus-envelope glycoproteins were detected at the cell surface. By 3 h after infection, cell surface membranes were extensively modified by the insertion of the Sindbis glycoproteins. When infected cells were prefixed with glutaraldehyde before labeling, the glycoproteins were distributed fairly evenly over the cell surface, although a slight clustering was observed on cells labeled early in infection. However, no evidence for large-scale clustering of virus glycoproteins corresponding to patches of budding virus was observed. Similar results were found with unfixed cells labeled at 4 C. However, when unfixed cells were labeled at 37 C, the glycoproteins were shown to be in discrete clusters, demonstrating that these glycoprotein antigens can diffuse laterally through the cell membrane at this temperature