| HMG-CoA reductase (HMGR), a highly conserved, membrane-bound enzyme, catalyzes a rate-limiting step in sterol and isoprenoid biosynthesis and is the primary target of hypocholesterolemic drug therapy. HMGR activity is tightly regulated to ensure maintenance of lipid homeostasis, disruption of which is a major cause of human morbidity and mortality. The general features of HMGR regulation, including a requirement for the HMGR-binding protein Insig, are remarkably conserved between mammals and fungi, including Schizosaccharomyces pombe.;Insig functions as a central regulator of mammalian cholesterol homeostasis by regulating HMGR. Insig both accelerates the degradation of HMGR and suppresses HMGR transcription. The fission yeast Schizosaccharomyces pombe encodes homologs of Insig and HMGR called ins1+ and hmg1+. In this thesis, I sought to determine whether Ins1 regulates Hmg1, and to reveal the mechanism of regulation. I show that Ins1 binding to Hmg1 inhibits enzyme activity by promoting phosphorylation of the Hmg1 active site. Furthermore, I show that Hmg1 activity is tightly regulated by glucose availability through Ins1-dependent phosphorylation, and that ablation of this system results in inappropriately high rates of sterol synthesis during glucose deprivation. Thus, I have described the first example of Insig function outside of lipid sensing, revealing a novel signal transduction pathway and a physiologically important mechanism by which cells can link the rate of sterol biosynthesis to glucose availability. |
