The other half of the
virus pellet was treated with glycerin to a final concentration
of 50%. Samples were incubated at 37°C for 15 min, diluted
to 13-ml volumes in PBS, and centrifuged at 100,000 x g in
an SW41 rotor for 1 h. The pellets were resuspended in a
0.5-ml volume of 3% paraformaldehyde. The samples were
layered onto continuous 10 to 50% potassium tartrate gradients
and centrifuged at 100,000 x g for 2 h.