Coordinate Regulation of Yeast Sterol Regulatory Element-binding Protein (SREBP) and Mga2 Transcription Factors

Risa Burr,Emerson V Stewart,Peter J Espenshade

doi:10.1074/jbc.m117.778209

Abstract

The Mga2 and Sre1 transcription factors regulate oxygen-responsive lipid homeostasis in the fission yeast Schizosaccharomyces pombe in a manner analogous to the mammalian sterol regulatory element-binding protein (SREBP)-1 and SREBP-2 transcription factors. Mga2 and SREBP-1 regulate triacylglycerol and glycerophospholipid synthesis, whereas Sre1 and SREBP-2 regulate sterol synthesis. In mammals, a shared activation mechanism allows for coordinate regulation of SREBP-1 and SREBP-2. In contrast, distinct pathways activate fission yeast Mga2 and Sre1. Therefore, it is unclear whether and how these two related pathways are coordinated to maintain lipid balance in fission yeast. Previously, we showed that Sre1 cleavage is defective in the absence of mga2 Here, we report that this defect is due to deficient unsaturated fatty acid synthesis, resulting in aberrant membrane transport. This defect is recapitulated by treatment with the fatty acid synthase inhibitor cerulenin and is rescued by addition of exogenous unsaturated fatty acids. Furthermore, sterol synthesis inhibition blocks Mga2 pathway activation. Together, these data demonstrate that Sre1 and Mga2 are each regulated by the lipid product of the other transcription factor pathway, providing a source of coordination for these two branches of lipid synthesis.

Highlights

The fission yeast sterol regulatory element-binding protein (SREBP)2 transcription factor Sre1 maintains sterol homeostasis and regulates a low oxygen-responsive transcriptional program [1, 2]
The Sre1 cleavage defects in both mga2⌬ and CER-treated cells were rescued by addition of exogenous 18:1, indicating that unsaturated fatty acid (UFA) regulates Sre1 cleavage and that a defect in fatty acid homeostasis is responsible for the Sre1 cleavage defect (Figs. 6, A and B, and 7C)
The Sre1 cleavage defect that results from CER treatment of WT cells was more fully rescued by UFA than that observed in the absence of mga2 (Fig. 6, A and B, versus Fig. 7C)

Summary

Results

Mga2⌬ Cells Have Reduced Sre1N Accumulation in the Presence and Absence of Oxygen—We previously screened the fission yeast deletion collection to identify regulators of Sre and found that mga deletion blocked Sre cleavage induction by low oxygen [9]. These data demonstrate that localization of the Golgi protein Anp and the ER protein Ost is altered in the absence of mga2 These two proteins are not components of the Sre pathway, suggesting that membrane transport is broadly defective in mga2⌬ cells. All of the fatty acid treatments significantly reduced Ost1mCherry intensity in the ER but not to wild-type levels (Fig. 6, C and E) These data suggest that defects in fatty acid homeostasis in mga2⌬ cells are responsible for the observed Sre cleavage and membrane transport defects. Treatment with CER induced Mga N terminus accumulation and decreased the precursor form (Fig. 8A) These results were consistent with data in S. cerevisiae showing that the proteasome is required for Mga cleavage and that Mga activation is regulated by unsaturated fatty acids [11, 12].

Discussion

Findings

Experimental Procedures