Fig. 1. CBP is associated with HMT activity, which methylates free histones and polynucleosomal substrates. (A) HeLa cell nuclear extracts (25 µl) were immunoprecipitated with the indicated antibody (anti-CBP: A-22, Santa Cruz; irrelevant: anti-p107, C-18, Santa Cruz). Immunoprecipitates were then assayed for HAT activity (left panel) or HMT activity (right panel) using purified histones as substrate. (B) As in (A) except that the CBP antibody (A-22) was pre-incubated with an increasing amount (2 and 5 µg) of its specific competitor peptide (peptide A-22 P, Santa Cruz) or of an irrelevant peptide (from P/CAF, Eurogentec) before being used for immunoprecipitation. As a control, immunoprecipitation with the irrelevant anti-p130 antibody (C-20, Santa Cruz) was performed. (C) As in (A) with 17.5 µl of nuclear extracts, except that polynucleosomes were used as substrates instead of histones. The HMT activity measured using purified histones was lower than in (A) due to variation from one batch of HeLa nuclear extracts to the other.