Leu628 of the KIX domain of CBP is a key residue for the interaction with the MLL transactivation domain.

Physical interaction between the transactivation domain (TAD) of the mixed-lineage leukemia protein (MLL) and the KIX domain of the cyclic-AMP response element binding protein (CREB) binding protein (CBP) is necessary for MLL-mediated transcriptional activation. We show by alanine-scanning mutagenesis that hydrophobic surface residues of KIX, especially L628, are energetically important for binding the MLL TAD. NMR studies of the KIX-L628A mutant suggest that L628 plays a crucial role in conformational transitions at the MLL binding site, necessary for high affinity interactions with MLL. Unexpectedly, MLL also binds to the c-Myb/phosphorylated kinase-inducible domain of CREB (pKID) site of KIX, highlighting the complex nature of interactions involving intrinsically disordered transcriptional activators.