Crystal Structure of the C-Terminal Domain of the Hypoxia Regulator Ofd1

Abstract

Sre1, the fission yeast homologue of the mammalian sterol regulatory element binding protein, SREBP, is a hypoxic transcription factor required for sterol homeostasis and low oxygen growth. The level of the N-terminal transcription factor domain of Sre1, Sre1N, is regulated by Ofd1-Nro1 in an oxygen-dependent manner. Ofd1 is a putative prolyl hydroxylase of the 2-oxyoglutarate-Fe(II) dioxygenase protein superfamily. Unlike the prolyl hydroxylase PHD which hydroxylates proline residues in the hypoxia-inducible factor HIF, Ofd1 utilizes its N-terminal dioxygenase domain to control the interaction between itself and its inhibitor Nro1: in hypoxia, Nro1 binds to Ofd1 and this inhibits its C-terminal degradation domain (CTDD) function on Sre1N destabilization. As shown previously, Ofd1CTDD by itself is sufficient to promote Sre1N degradation and there's direct interaction between Nro1 and Ofd1CTDD. However, the molecular mechanism by which Ofd1CTDD accelerates Sre1N degradation remains unclear. Here we report the crystal structure of Ofd1CTDD at 2.0 Å resolution. Interestingly, Ofd1CTDD has a double-stranded beta helix (DBSH) fold like that of the 2-oxyoglutarate-Fe(II) dioxygenase protein superfamily but lacks the iron-binding motif characteristic of the superfamily. This structure may help elucidate how Ofd1CTDD promotes Sre1N turnover and how Ofd1-Nro1 as a whole functions as an oxygen-sensor.