Abstract
Sindbis virus was adsorbed to chicken cells or to BHK cells, and the distribution of virus over the surface of the cell was examined by electron microscopy of surface replicas. The distribution of virus particles on the cell was used to indicate the position of virus receptors at the cell surface. When purified Sindbis virus was adsorbed at 37 C to cells prefixed with glutaraldehyde, the virus particles were evenly distributed over the surface of most cells. There was a large variability from cell to cell, however, in the number of virus particles adsorbed, and regions with different concentrations of virus particles were sometimes observed on the same cell. The concentration of virus receptors observed varied from 20 to 160/mum(2) of cell surface, and, thus, the total number of virus receptors per chicken cell is on the order of 10(5). When virus was adsorbed to unfixed cells at 4 C, the virus particles were clustered into aggregates varying from a few particles to large crystalline arrays (the latter seen only in chicken cells). These conditions are apparently conducive to virus aggregation, and this, coupled with free lateral diffusion of the virus-receptor complex in the cell membrane at 4 C, leads to the observed clustering.