Fig. 6. The IκBα phosphopeptide specifically inhibits conjugation and processing of p105 in vitro. (A) The IκBα phosphopeptide inhibits conjugation of p105. Conjugation of p105 in a cell-free system was monitored as described in Materials and methods in the absence or presence of ATP, the IκBα-derived phosphopeptide (pP) or the S32,36A control peptide (S/A) as indicated. (B) The IκBα phosphopeptide inhibits processing of p105. Processing of p105 in a cell-free system was monitored as described in Materials and methods and in (A). (C) Effect of the IκBα-derived phosphopeptide on processing of wild-type p105, p105-Δ918–934 and p105-Δ446–454. Processing of wild-type p105, p105-Δ918–934 and p105-Δ446–454 was monitored in a cell-free system as described in Materials and methods in the absence or presence of ATP and the IκBα-derived phosphopeptide (pP), as indicated.