Functional analysis of hypoxia-inducible factor-1α-mediated transactivation -: identification of amino acid residues critical for transcriptional activation and/or interaction with CREB-binding protein

The hypoxia-inducible factor-1alpha (HIF-1alpha) is a key regulator of adaptive responses to hypoxia. HIF-1alpha has two independent transactivation domains (TADs). Whereas the N-terminal TAD (N-TAD) also constitutes a degradation box, the C-terminal TAD (C-TAD) functions in a strictly hypoxia-inducible fashion. Oxygen-dependent hydroxylation of an asparagine residue has recently been reported to regulate C-TAD function by disrupting the interaction with the CH1 domain of the p300/CBP coactivator at normoxia. Here we have performed alanine-scanning mutagenesis of a predicted alpha-helix within the C-TAD of mouse HIF-1alpha to identify residues important for transactivation and interaction of the C-TAD with transcriptional coactivators. We observed that several hydrophobic residues, Ile(802), Leu(808), Leu(814), Leu(815), and Leu(818), were critical for transactivation and binding to the CH1 domain of CBP in hypoxic cells. Moreover, E812A/E813A and D819A mutations impaired hypoxia-dependent transactivation without disrupting binding to CH1. In the context of full-length HIF-1alpha, mutation of the leucine residues conferred conformational changes to the protein and significantly reduced the transactivation function as well as functional interaction with the transcriptional coactivators CBP and SRC-1. These mutations also affected intranuclear redistribution of HIF-1alpha in the presence of CBP, indicating that the integrity of the C-TAD is critical for intracellular localization of mouse HIF-1alpha.