FIG. 3.
Fractionation of virus on a Percoll gradient suggests that a substantial amount of envelope does not cosediment with p24 after heat treatment. After ultrafiltration, HIVSX was either held at 4°C or heated to 62°C for 10 min. Next, 200 μl of virus was layered onto 1.8 ml of undiluted Percoll (Pharmacia), and the samples were centrifuged at 56,000 × g for 60 min at 4°C. Fractions (100 μl) were removed from the top of the gradient. The samples which were previously held at 4°C were then heated to 62°C for 10 min to normalize OD readings for the ELISA. HIV p24 was measured by capture ELISA (Coulter); gp120 ELISA was as described in the text using CD4-IgG as the capture antibody.