Conformational propensities of intrinsically disordered proteins influence the mechanism of binding and folding

Significance Intrinsically disordered proteins play a central role in cellular signaling and regulatory networks. Many regulatory proteins contain disordered recognition motifs that fold upon binding to their cellular targets, by mechanisms that are poorly understood. Here we show that the disordered transactivation domain of the transcription factor c-Myb binds to the KIX domain of the transcriptional coactivator cAMP-response element binding (CREB-binding) protein (CBP) by a mechanism that involves elements of conformational selection and induced fit. In contrast, the phosphorylated kinase-inducible activation domain of CREB binds to the same site on KIX by an induced-fit mechanism. The intrinsic secondary structure propensities of these two disordered proteins determine their binding mechanisms, consistent with their functions as inducible and constitutive transcriptional activators. Intrinsically disordered proteins (IDPs) frequently function in protein interaction networks that regulate crucial cellular signaling pathways. Many IDPs undergo transitions from disordered conformational ensembles to folded structures upon binding to their cellular targets. Several possible binding mechanisms for coupled folding and binding have been identified: folding of the IDP after association with the target (“induced fit”), or binding of a prefolded state in the conformational ensemble of the IDP to the target protein (“conformational selection”), or some combination of these two extremes. The interaction of the intrinsically disordered phosphorylated kinase-inducible domain (pKID) of the cAMP-response element binding (CREB) protein with the KIX domain of a general transcriptional coactivator CREB-binding protein (CBP) provides an example of the induced-fit mechanism. Here we show by NMR relaxation dispersion experiments that a different intrinsically disordered ligand, the transactivation domain of the transcription factor c-Myb, interacts with KIX at the same site as pKID but via a different binding mechanism that involves elements of conformational selection and induced fit. In contrast to pKID, the c-Myb activation domain has a strong propensity for spontaneous helix formation in its N-terminal region, which binds to KIX in a predominantly folded conformation. The C-terminal region of c-Myb exhibits a much smaller helical propensity and likely folds via an induced-fit process after binding to KIX. We propose that the intrinsic secondary structure propensities of pKID and c-Myb determine their binding mechanisms, consistent with their functions as inducible and constitutive transcriptional activators.

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