From the Department of Cell Biology, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21205.
J Biol Chem. 2014 Jan 31;289(5):2725-35. doi: 10.1074/jbc.M113.511899. Epub 2013 Dec 10.
Sterol homeostasis is tightly controlled by the sterol regulatory element-binding protein (SREBP) transcription factor that is highly conserved from fungi to mammals. In fission yeast, SREBP functions in an oxygen-sensing pathway to promote adaptation to decreased oxygen supply that limits oxygen-dependent sterol synthesis. Low oxygen stimulates proteolytic cleavage of the SREBP homolog Sre1, generating the active transcription factor Sre1N that drives expression of sterol biosynthetic enzymes. In addition, low oxygen increases the stability and DNA binding activity of Sre1N. To identify additional signals controlling Sre1 activity, we conducted a genetic overexpression screen. Here, we describe our isolation and characterization of the casein kinase 1 family member Hhp2 as a novel regulator of Sre1N. Deletion of Hhp2 increases Sre1N protein stability and ergosterol levels in the presence of oxygen. Hhp2-dependent Sre1N degradation by the proteasome requires Hhp2 kinase activity, and Hhp2 binds and phosphorylates Sre1N at specific residues. Our results describe a role for casein kinase 1 as a direct regulator of sterol homeostasis. Given the role of mammalian Hhp2 homologs, casein kinase 1δ and 1ε, in regulation of the circadian clock, these findings may provide a mechanism for coordinating circadian rhythm and lipid metabolism.
甾醇稳态受到甾醇调节元件结合蛋白（SREBP）转录因子的严格控制，该因子从真菌到哺乳动物高度保守。在裂殖酵母中，SREBP 在线粒体氧感应途径中发挥作用，促进适应限制依赖氧的甾醇合成的低氧供应。低氧刺激 SREBP 同源物 Sre1 的蛋白水解切割，产生活性转录因子 Sre1N，驱动甾醇生物合成酶的表达。此外，低氧增加了 Sre1N 的稳定性和 DNA 结合活性。为了鉴定控制 Sre1 活性的其他信号，我们进行了遗传过表达筛选。在这里，我们描述了我们如何分离和鉴定酪蛋白激酶 1 家族成员 Hhp2 作为 Sre1N 的新型调节剂。在有氧条件下，缺失 Hhp2 会增加 Sre1N 蛋白稳定性和麦角固醇水平。Hhp2 依赖的蛋白酶体降解 Sre1N 需要 Hhp2 激酶活性，并且 Hhp2 结合并在特定残基上磷酸化 Sre1N。我们的结果描述了酪蛋白激酶 1 作为甾醇稳态的直接调节剂的作用。鉴于哺乳动物 Hhp2 同源物（酪蛋白激酶 1δ 和 1ε）在调节生物钟中的作用，这些发现可能为协调昼夜节律和脂质代谢提供了一种机制。
