Sterol regulatory element-binding proteins (SREBPs) are membrane-bound transcription factors that promote lipid synthesis in animal cells. They are embedded in the membranes of the endoplasmic reticulum (ER) in a helical hairpin orientation and are released from the ER by a two-step proteolytic process. Proteolysis begins when the SREBPs are cleaved at Site-1, which is located at a leucine residue in the middle of the hydrophobic loop in the lumen of the ER [1]. Upon proteolytic processing SREBP can activate the expression of genes involved in cholesterol biosynthesis and uptake. SCAP stimulates cleavage of SREBPs via fusion of the their two C-termini [2]. This domain is the transmembrane region that traverses the membrane eight times and is the sterol-sensing domain of the cleavage protein [4]. WD40 domains are found towards the C-terminus.

1: Cleavage of sterol regulatory element-binding proteins (SREBPs) at site-1 requires interaction with SREBP cleavage-activating protein. Evidence from in vivo competition studies. Sakai J, Nohturfft A, Goldstein JL, Brown MS; J Biol Chem. 1998;273:5785-5793. PMID:9488713

2: Endoplasmic reticulum stress causes the activation of sterol regulatory element binding protein-2. Colgan SM, Tang D, Werstuck GH, Austin RC; Int J Biochem Cell Biol. 2007;39:1843-1851. PMID:17604677

3: Identification of complexes between the COOH-terminal domains of sterol regulatory element-binding proteins (SREBPs) and SREBP cleavage-activating protein. Sakai J, Nohturfft A, Cheng D, Ho YK, Brown MS, Goldstein JL; J Biol Chem. 1997;272:20213-20221. PMID:9242699

4: Three mutations in sterol-sensing domain of SCAP block interaction with insig and render SREBP cleavage insensitive to sterols. Yabe D, Xia ZP, Adams CM, Rawson RB; Proc Natl Acad Sci U S A. 2002;99:16672-16677. PMID:12482938