Anti-SREBP-1 from rabbit, purified by affinity chromatography

Sterol regulatory element-binding protein 1 (UniProt: P36956; also known as SREBP-1, Class D basic helix-loop-helix protein 1, bHLHd1, Sterol regulatory element-binding transcription factor 1) is encoded by the SREBF1 (also known as BHLHD1, SREBP1) gene (Gene ID: 6720) in human. SREBP-1 is a multi-pass membrane protein that acts as at transcriptional activator required for lipid homeostasis. Six isoforms of SREBP-1 have been described that are produced by alternative splicing. SREBP-1 is expressed in a wide variety of tissues and is most abundant in liver and adrenal gland. In fetal lung and liver it is expressed in high levels. SREBP-1A predominates in hepatoma cell lines while isoform SREBP-1C is predominantly found in liver, adrenal gland, and ovary. Both isoforms IA and IC is found in kidney, brain, white fat, and muscle. SREBP-1 regulates transcription of the LDL receptor gene as well as the fatty acid and to a lesser degree the cholesterol synthesis pathway. It binds to the sterol regulatory element 1 (SRE-1) (5'-ATCACCCCAC-3') and has dual sequence specificity binding to both an E-box motif (5'-ATCACGTGA-3') and to SRE-1 (5'-ATCACCCCAC-3'). SREBP-1 has two cytoplasmic domains (aa 1-487 and 567-1147; two helical domains (aa 488-508 and 548-568), and a luminal domain (aa 509-547). In the absence of sterols, it moves from the endoplasmic reticulum to the Golgi. At low cholesterol the SCAP/SREBP complex is recruited into COPII vesicles for export from the ER. In the Golgi complex SREBPs are cleaved sequentially by site-1 and site-2 protease. The first cleavage by site-1 protease occurs within the luminal loop, the second cleavage by site-2 protease occurs within the first transmembrane domain and releases the transcription factor from the Golgi membrane.