FIG. 3.

Capacity of r-cores+ς1 to bind to cells. (a) HA induced by purified virions, cores, r-cores, and r-cores+ς1 was determined by endpoint titration using human type A⁺ RBCs (American Red Cross), as described previously (20). Serial twofold virus dilutions were incubated with RBCs (0.4% [vol/vol]) in a 96-well round-bottomed plate at 4°C for 2 h, and HA was visually detected. The number of HA units induced by a virus preparation is given by the highest number of particles used (2 × 10¹⁰ particles) divided by the minimum number of particles required to induce HA. HA activity is expressed as the log₂(HA units). Averages ± standard deviations of three replicates are shown. (b) Attachment of purified virions, r-cores, and r-cores+ς1 to L cells at 4°C was determined by indirect immunofluorescence as described previously (18). Cells on coverslips were incubated with purified virus (5 × 10⁵ particles/cell) at 4°C for 1 h. Unbound virus was removed by washing with ice-cold phosphate-buffered saline. After the cells were fixed and permeabilized, bound virus was detected by using a 1:500 dilution of rabbit antireovirus serum (44) as primary antibody and a 1:100 dilution of fluorescein isothiocyanate-conjugated donkey anti-rabbit immunoglobulin G (Pierce) as secondary antibody.