PDI acts on Env prior to the 6-helix bundle conformation of gp41. Sup T-1 cells were treated with 0 to 5 mM DTNB. DTNB-treated cells were then cocultured with Env⁺ cells in the presence of anti–6 helix bundle PAb for 1 hour at 37°C. The cells were lysed, incubated with Protein G-coupled beads, then immobilized proteins were analyzed for gp41 content by SDS-PAGE and immunoblotting with anti-gp41 MAb (Chessie 8) (A). As a control, residual supernatants from immunoprecipitation were analyzed for actin content with an anti-actin PAb (B). Data shown are representative of 2 experiments.