Analysis of the Cell and Erythrocyte Binding Activities of the Dimple and Canyon Regions of the Canine Parvovirus Capsid

Canine parvovirus (CPV) binds to a number of cell and erythrocyte receptors, some of which are involved in cell infection, while others are used for other viral functions. Little is known about the regions of the virus capsid which bind to the cell receptors. CPV binds sialic acid through a region within or adjacent to the dimple on the surface of the capsid (Barbis, D. P., Chang, S-F., and Parrish, C. R., 1992, Virology 191, 301-308). In order to map the sialic acid binding site in more detail and to examine other regions of the capsid for cell receptor binding, a variety of mutant capsids were analyzed which had changes in two depressions within the surface of the capsid--the "canyon" and "dimple." In most cases recombinant VP1 and VP2 proteins were stably expressed together in canine A72 cells from a plasmid expression vector. The purified empty capsids were tested for their ability to bind sialic acid and thereby hemagglutinate (HA) erythrocytes and for binding to permissive host cells. In addition, the ability of neutralizing monoclonal antibodies to block cell attachment was also examined. Mutations of amino acids on a wall of the dimple eliminated or severely decreased HA. Changing various residues within the canyon had no effect on binding to either sialic acids or other receptors on feline lymphoblastoid cells, suggesting that the canyon is not the site of cell receptor attachment. Neutralizing monoclonal antibodies against both major antigenic determinants had variable effects on cell binding, but no consistent inhibition of binding was observed by antibodies directed against either of those two major antigenic determinants of the capsid.