FIG. 7.

Virus density gradient fractionation. Samples of the indicated viruses were loaded onto 20-to-60% sucrose density gradients and centrifuged at 170,000 × g (average) for 18 h in an SW 50.1 rotor such that particles of ≥12S should have sedimented to equilibrium. After centrifugation, 0.4-ml fractions were collected from the gradient tops (A) to bottoms (M) and Gag levels in each fraction were measured by densitometry of immunoblot bands. Gag levels for each fraction are plotted relative to the gradient fraction with the highest Gag signal (=100%). Sucrose densities were determined by weighing a constant volume of each fraction from a gradient run in parallel, although these values should be considered approximate, as our observed gradient-to-gradient variation was about ±0.005 g/ml in any given fraction.