Interaction of the TAZ1 Domain of the CREB-Binding Protein with the Activation Domain of CITED2

The TAZ1 domain of the homologous transcriptional coactivators CREB-binding protein (CBP) and p300 forms a complex with CITED2 (CBP/p300-interacting transactivator with ED-rich tail), inhibiting the activity of the hypoxia inducible factor (HIF-1α) and thereby attenuating the cellular response to low tissue oxygen concentration. We report the NMR structure of the CBP TAZ1 domain bound to the activation domain of CIT-ED2. The structure of TAZ1, consisting of four α-helices (α1-α4) stabilized by three zinc atoms, is very similar in the CITED2 and HIF-1α complexes. The activation domain of CITED2 is unstructured when free and folds upon binding, forming a helix (termed αA) and an extended structure that wraps around TAZ1. The CITED2 αA helix packs in the TAZ1 α1/α4 interface, a site that forms weak interactions with the poorly defined aminoterminal α-helix of HIF-1α. CITED2 and HIF-1α both contain a four residue motif, LP(E/Q)L, which binds in the TAZ1 α1/α2/α3 junction in each complex. The carboxyl-terminal region of CITED2 forms an extended structure with hydrophobic contacts in the TAZ1 α1/α3 interface in the site occupied by the HIF-1α αB helix. CITED2 does not bind at all to the TAZ1 site occupied by the HIF-1α carboxyl-terminal helix. The HIF-1α and CITED2 domains utilize partly overlapping surfaces of TAZ1 to achieve high affinity binding and to compete effectively with each other for interaction with CBP/p300; CITED2 and HIF-1α use these binding sites differently to maintain similar binding affinities in order to displace each other in a feedback loop during the hypoxic response.