Direct binding to sterols accelerates endoplasmic reticulum- associated degradation of HMG CoA reductase

The maintenance of cholesterol homeostasis is crucial for normal function at both the cellular and organismallevels. Two integral membrane proteins, 3- hydroxy-3- methylglutaryl coenzyme A reductase (HMGCR) and Scap, are key targets of a complex feedback regulatory system that operates to ensure cholesterol homeostasis. HMGCR catalyzes the rate- limiting step in the transformation of the 2- carbon precursor acetate to 27- carbon cholesterol. Scap mediates proteolytic activation of sterol regulatory element- binding protein - 2 (SREBP-2), a membrane - bound transcription factor that controls expression of genes involved in the synthesis and uptake of cholesterol. Sterol accumulation triggers binding of HMGCR to endoplasmic reticulum (ER)- localized Insig proteins, leading to the enzyme's ubiquitination and proteasome- mediated ER- associated degradation (ERAD). Sterols also induce binding of Insigs to Scap, which leads to sequestration of Scap and its bound SREBP-2 in the ER, thereby preventing proteolytic activation of SREBP-2 in the Golgi. The oxygenated cholesterol derivative 25- hydroxycholesterol (25HC) and the methylated cholesterol synthesis intermediate 24,25- dihydrolanosterol (DHL) differentially modulate HMGCR and Scap. While both sterols promote binding of HMGCR to Insigs for ubiquitination and subsequent ERAD, only 25HC inhibits the Scap- mediated proteolytic activation of SREBP-2. We showed previously that 1,1- bisphosphonate esters mimic DHL, accelerating ERAD of HMGCR while sparing SREBP-2 activation. Building on these results, our current studies reveal specific, Insig- independent photoaffinity labeling of HMGCR by photoactivatable derivatives of the 1,1- bisphosphonate ester SRP-3042 and 25HC. These findings disclose a direct sterol binding mechanism as the trigger that initiates the HMGCR ERAD pathway, providing valuable insights into the intricate mechanisms that govern cholesterol homeostasis.