Precursor of the transcription factor form (Processed sterol regulatory element-binding protein 1), which is embedded in the endoplasmic reticulum membrane (PubMed:11782483, PubMed:12855691, PubMed:19244231). Low sterol concentrations promote processing of this form, releasing the transcription factor form that translocates into the nucleus and activates transcription of genes involved in cholesterol biosynthesis and lipid homeostasis (PubMed:11782483, PubMed:12855691, PubMed:16100574, PubMed:19244231). Key transcription factor that regulates expression of genes involved in cholesterol biosynthesis and lipid homeostasis (PubMed:19244231, PubMed:17290224, PubMed:9329978, PubMed:9784493, PubMed:21459323). Binds to the sterol regulatory element 1 (SRE-1) (5'-ATCACCCCAC-3') (By similarity). Has dual sequence specificity binding to both an E-box motif (5'-ATCACGTGA-3') and to SRE-1 (5'-ATCACCCCAC-3') (By similarity). Regulates the promoters of genes involved in cholesterol biosynthesis and the LDL receptor (LDLR) pathway of sterol regulation (PubMed:19244231, PubMed:17290224, PubMed:9329978, PubMed:9784493, PubMed:21459323). Isoform expressed only in select tissues, which has higher transcriptional activity compared to SREBP-1C (PubMed:12855691, PubMed:21531336). Able to stimulate both lipogenic and cholesterogenic gene expression (PubMed:8833906). Has a role in the nutritional regulation of fatty acids and triglycerides in lipogenic organs such as the liver (PubMed:9062341, PubMed:12855691). Required for innate immune response in macrophages by regulating lipid metabolism (PubMed:21531336). Predominant isoform expressed in most tissues, which has weaker transcriptional activity compared to isoform SREBP-1A (PubMed:12855691, PubMed:21531336). Primarily controls expression of lipogenic gene (PubMed:8833906, PubMed:9062341). Strongly activates global lipid synthesis in rapidly growing cells (PubMed:8833906, PubMed:9062341). Forms a tight complex with SCAP, the SCAP-SREBP complex, in the endoplasmic reticulum membrane. Efficient DNA binding of the soluble transcription factor fragment requires dimerization with another bHLH protein (By similarity). Interacts with CEBPA, the interaction produces a transcriptional synergy (PubMed:17290224). Interacts with LMNA (PubMed:11929849). At high sterol concentrations, the SCAP-SREBP is retained in the endoplasmic reticulum (PubMed:21459323). Low sterol concentrations promote recruitment into COPII-coated vesicles and transport of the SCAP-SREBP to the Golgi, where it is processed (PubMed:21459323). Additional isoforms seem to exist. Predominant isoform expressed in most tissues (PubMed:21531336). Predominates in liver, adrenal gland, brain and adipose tissue (PubMed:9062340). Also found in kidney, thymus, testis, muscle, jejunum, and ileum (PubMed:9062340). Expressed only in select tissues, such as intestinal epithelial, heart, macrophage and bone marrow dendritic cells (PubMed:9062340, PubMed:21531336). Also found in kidney, thymus, testis, muscle, jejunum, and ileum (PubMed:9062340). Expressed in a circadian manner in the liver with a peak at ZT16 (PubMed:19786558). Up-regulated by endocannabinoid anandamide/AEA. The 9aaTAD motif is a transactivation domain present in a large number of yeast and animal transcription factors. Processed in the Golgi apparatus, releasing the protein from the membrane. At low cholesterol the SCAP-SREBP complex is recruited into COPII vesicles for export from the endoplasmic reticulum. In the Golgi, complex SREBPs are cleaved sequentially by site-1 (MBTPS1, S1P) and site-2 (MBTPS2, S2P) proteases (By similarity). 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, releasing the transcription factor from the Golgi membrane (By similarity). Phosphorylated by AMPK, leading to suppress protein processing and nuclear translocation, and repress target gene expression. Phosphorylation at Ser-389 by SIK1 represses activity possibly by inhibiting DNA-binding. Ubiquitinated; the nuclear form has a rapid turnover and is rapidly ubiquitinated and degraded by the proteasome in the nucleus. Mice show high embryonic lethality around day 11 dpc (PubMed:9329978). Surviving mice show a 2-3-fold increase in processed Srebpf2 protein in liver nuclei, 3-fold increase in cholesterol synthesis and 50% increase in cholesterol content of the liver (PubMed:9329978). Mice lacking isoform SREBP-1A are resistant to pro-inflammatory toxic shock (PubMed:21531336). Macrophages challenged with bacterial lipopolysaccharide fail to activate lipogenesis as well as hallmarks of inflammasome functions, activation of caspase-1 and secretion of IL1B (PubMed:21531336). Mice lacking isoform SREBP-1C show a lack of up-regulation of several lipogenic enzymes in response to high insulin or LXR activation. Belongs to the SREBP family.