Sterol-sensing domain of SREBP cleavage-activation
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. 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. This domain is the transmembrane region that traverses the membrane eight times and is the sterol-sensing domain of the cleavage protein. WD40 domains are found towards the C-terminus.