A variant of nuclear localization signal of bipartite-type is required for the nuclear translocation of hypoxia inducible factors (1α, 2α and 3α)

Hypoxia inducible factors (HTF1, 2 and 3), consisting of alpha and beta subunits, play an essential role in various responses to hypoxia. Nuclear entry of alpha subunits is a necessary step for the formation of DNA-binding complex with beta subunit, which is constitutively localized in the nucleus. We show here that the nuclear accumulation of HIF2 alpha induced by hypoxia is mediated through a novel variant of bipartite-type nuclear localization signal (NLS) in the C-terminus of the protein, which has an unusual length of spacer sequence between two adjacent basic domains. We further show that when the ubiquitin-proteasome system was deficient or inhibited, HIF2a accumulated in the nucleus even under normoxia, also mediated through the bipartite NLS, These findings indicate that the protein stability is critical for the nuclear localization of HIF2a and hypoxia is not a necessary factor for the process. Importantly, the NLS of HIF2a is also conserved in the other HIF family members, HIF1 alpha and HIF3 alpha. Mutational analyses proved that the NLS mediating the nuclear localization of HIF1 alpha is indeed bipartite-, but not monopartite-type as thought before, Our results suggest that the newly identified NLS is crucial for the functional regulation of HIF family.