Transport of hypoxia-inducible factor HIF-1α into the nucleus involves importins 4 and 7

Hypoxia-inducible transcription factor 1 (HIF-1) mediates the cellular response to hypoxia. HIF-1 activity is controlled via the synthesis, degradation or intracellular localization of its α subunit. HIF-1α contains a C-terminal bipartite basic NLS that interacts with importins α. We have recently shown that HIF-1α also contains an atypical hydrophobic CRM1- and phosphorylation-dependent NES and can therefore shuttle in and out of the nucleus. We now report that C-terminal NLS mutants of HIF-1α can still enter the nucleus when CRM1-dependent nuclear export is inhibited, indicating that HIF-1α contains an additional functional nuclear import signal. Using an in vitro nuclear import assay, we further show that importins 4 and 7 accomplish nuclear import of HIF-1α more efficiently than the classical importin α/β NLS receptor. Binding assays confirmed the specific physical interaction between HIF-1α and importins 4 and 7. Moreover, the interaction of importin 7 with HIF-1α is mapped at its N-terminal part encompassing the bHLH–PASA domain. By expressing functional HIF-1 in yeast, we show that Nmd5, the yeast orthologue of importin 7, is required for HIF-1α nuclear accumulation and activity. Taken together, our data show that shuttling of HIF-1α between cytoplasm and nucleus is a complex process involving several members of the nuclear transport receptor family.