Fig. 5. Interaction of Tax with p100 via two α helices, which is correlated with Tax-induced p100 processing. (A) Scheme of p100 indicating the two α helices, αA and αB, previously identified by X-ray crystallography (Cramer et al., 1997). (B) 293 cells were transfected with Tax together with the indicated p100 constructs. The p100 proteins were isolated from cell lysates by IP followed by detection of the coprecipitated Tax by IB (upper panel). The cell lysates were also subjected to IB to monitor the p100 processing (middle panel) and Tax expression (lower panel). (C) Dimerization of p100 mutants with the Rel homology domain of RelA, RelA(1–312). 293 cells were transfected with RelA(1–312) together with the p100 constructs, followed by coIP assays.