The mechanism of binding of the KIX domain to the mixed lineage leukemia protein and its allosteric role in the recognition of c-Myb

The KIX domain is a mediator of the interaction between different transcription factors. This complex function is carried out via two distinct binding sites located on opposite sides of the protein; namely, the c-Myb site' and the MLL site', named after their characteristic ligandsthe transactivation domain of c-Myb and the mixed lineage leukemia protein (MLL). Both these ligands are unstructured in isolation and fold only upon binding, posing the KIX domain as an ideal candidate to explore the binding induced folding reaction of intrinsically unstructured proteins. Here, we complement the recent kinetic description on the interaction between KIX and c-Myb, by characterizing the binding kinetics between KIX and MLL, at different pH and ionic strength conditions. Furthermore, we analyze quantitatively the mechanism of allosteric communication between the topologically distinct c-Myb and MLL sites. The implications of our results are discussed in the light of previous work on other intrinsically unstructured systems.