Sindbis virus-infected baby hamster kidney cells were analyzed by thin section fracture-label. Specific immunolabel with antiviral glycoprotein antibodies or with conventional lectin label (wheat germ agglutinin) were used in conjunction with colloidal gold-conjugated protein A or ovomucoid, respectively. In addition, intact infected cells were analyzed with both labeling procedures. Experiments with Sindbis infected-chick embryo fibroblast cells were carried out as controls. Viral transmembrane glycoproteins appeared present in freeze-fractured inner and outer nuclear membrane, endoplasmic reticulum, Golgi stacks and vesicles, and plasma membranes; a clear preferential partition with the exoplasmic faces of all intracellular membranes was observed. By contrast, at the plasma membrane level, Sindbis glycoproteins were found to partition preferentially with the protoplasmic face. It seems likely that this protoplasmic partition is related to the binding with the nucleocapsid that takes place during the budding of the virus. At the cell surface, viral glycoproteins always appeared clustered and were predominantly associated with budding figures: moreover, large portions of the plasma membrane were devoid of both glycoproteins and budding viruses.

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