Oxygen‐Dependent Degradation of Yeast SREBP Requires the N‐End Rule E3 Ligase Ubr1

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Eukaryotic cells experience fluctuating environmental oxygen concentrations and elicit a physiological response to allow the organism or tissue to adapt. Sre1, the fission yeast Sterol Regulatory Element Binding Protein, is an ER membrane‐bound transcription factor that responds to changes in oxygen‐dependent sterol synthesis as an indirect measure of oxygen availability. Under low oxygen, Sre1 is proteolytically cleaved and the released N‐terminal transcription factor (Sre1N) activates gene expression essential for hypoxic growth. In addition, Sre1N turnover is regulated by oxygen. Ofd1, a prolyl 4‐hydroxylase family member, accelerates Sre1N degradation in the presence of oxygen. However, unlike the prolyl 4‐hydroxylases that regulate mammalian hypoxia inducible factor (HIF), Ofd1 uses multiple domains to regulate Sre1N degradation by oxygen; the Ofd1 N‐terminal dioxygenase domain is required for oxygen sensing and the Ofd1 C‐terminal domain accelerates Sre1N degradation. Proteasomal degradation of Sre1N requires the E2 ubiquitin conjugating enzyme Rhp6 and the N‐end rule E3 ligase Ubr1. Genetic experiments position Ofd1 in this pathway upstream of Ubr1. Thus, oxygen regulates Sre1 activity at two points to mediate adaptation to hypoxic stress: (1) proteolytic cleavage and (2) transcription factor degradation. Research was supported by the NIH‐HL‐077588 and NIH‐AI‐072186.

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