Two most important proteins regulating cholesterol biosynthesis are SREBP and SCAP. SREBPs (sterol regulatory element-binding proteins) control lipid synthesis in nonhepatic cells. SCAP (SREBP cleavage-activating protein) is a sterol-regulated escort protein transporting SREBPs from their site of synthesis in the endoplasmic reticulum to their site of cleavage in the Golgi apparatus (1). Once SREBPs are synthesized as ER membrane proteins they bind with SREBP to form the SREBP-SCAP complex that rtains in the ER. When sterol levels in the body drop, the SREBP-SCAP complex is transported to the Golgi where proteolytic cleavage of SREBP releases the N-terminal bHLH domain, allowing nuclear translocation and increased expression of target genes.
To activate lipid biosynthesis, SREBPs must be transported from the endoplasmic reticulum (ER) to the Golgi apparatus, in which they are cleaved sequentially by two proteases that release the cytosolic NH2-terminal transcription factor domains. When the fragments of the SREBPs are cleaved sequentially by two proteases in the Golgi apparatus, they then enter the nucleus and bind to sterol regulatory elements in the enhancer regions of genes responsible for mediation of the synthesis of cholesterol and unsaturated fatty acids.
