The first step in virus replication is attachment of the virus to the host cell surface. To investigate this process, the binding of TC-83 (the attenuated strain of Venezuelan equine encephalomyelitis virus) to BW-J-M (a macrophage-like murine cell culture line) has been characterized. The binding of radiolabelled virus can be blocked by excess unlabelled virus and has a pH optimum in the physiological range. Binding is saturable; analysis using Scatchard plots or the computerized binding data analysis program, LIGAND, yielded estimates of a single class of 4 × 10 binding sites per cell and an equilibrium binding constant of 2·0 ± 0·7 × 10 . Virus bound to the cell could be visualized by transmission electron microscopy and was localized primarily in coated regions of the membrane. Virus, bound under optimum conditions at 0 °C, was internalized upon warming and infected cells productively. Treatment of BW-J-M cells with low concentrations (1 to 10 µg/ml) of the proteolytic enzymes trypsin, Pronase or proteinase K caused a dose-dependent reduction in binding capacity. Trypsin-treated cells, upon return to culture, progressively regained their binding capacity within 24 h. As a further characterization of the virus binding site, several lectins were studied for their ability to inhibit TC-83 binding to BW-J-M cells. agglutinin, agglutinin and agglutinin were potent inhibitors of virus binding. This evidence suggests that TC-83 binds to a specific receptor on the BW-J-M cell and that the receptor may be glycoprotein.


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