A large body of evidence suggests that a considerable fraction of the human proteome may be at least in part intrinsically unstructured. While disordered, intrinsically unstructured proteins are nevertheless functional and mediate many interactions. Despite their significant role in regulation, however, little is known about the molecular mechanism whereby intrinsically unstructured proteins exert their function. This basic problem is critical to establish the role, if any, of disorder in cellular systems. Here we present kinetic experiments supporting a mechanism of binding-induced-folding when the KIX domain of the CREB-binding protein binds the transactivation domain of c-Myb, an intrinsically unstructured domain. The high-resolution structure of this physiologically important complex was previously determined by NMR spectroscopy. Our data reveal that c-Myb recognizes KIX by first forming a weak encounter complex in a disordered conformation, which is subsequently locked-in by a folding step, i.e. binding precedes folding. On the basis of the pH dependence of the observed combination and dissociation rate constants we propose a plausible mechanism for complex formation. The implications of our results in the light of previous work on intrinsically unstructured systems are discussed. (C) 2012 Elsevier Inc. All rights reserved.