SREBP-1 and -2 (sterol-regulatory element binding proteins-1 and -2) are transcription factors which participate in the control of cholesterol homeostasis. SREBPs proteins, which are attached to the endoplasmic reticulum and nuclear envelope, are proteolytically cleaved and thus activated in response to conditions of low cellular sterol. Upon activation of SREBP-1 or -2, an ~480-500 amino acid, N-terminal cleavage fragment of these proteins enters the nucleus and activates transcription of enzymes and other proteins required for cholesterol synthesis. Proteases which cleave SREBPs have been identified and include SCA (SREBP-cleavage activity), as well as a key regulator of apoptotic pathways, caspase-3. SREBP proteins containing point mutations at caspase-3 cleavage sites (Asp460 in SREBP-1 and Asp468 in SREBP-2) do not become cleaved following induction of apoptosis, suggesting that SREBPs may play some role in apoptotic processes. However, sterol-regulated vs. apoptosis-associated cleavage of SREBP proteins appears to be independantly regulated. On SDS-PAGE, sterol-regulated cleavage fragments of SREBP proteins migrate more slowly (i.e., higher molecular weight) than do staurosporin-induced fragments. In addition, staurosporin-induced SREBP cleavage products may appear as a doublet, with the upper band representing a phosphorylated form of SREBP. On SDS-PAGE, full length, precursor forms of SREBP-1 and -2 migrate at ~125 kDa, while proteolytic cleavage fragments may be observed as a cluster of bands between 60 - 70 kDa. The IgG-2A4 antibody recognizes human and hamster SREBP-1. The antibody recognizes the N-terminal (basic helix-loop-helix) domain of human SREBP-1. A fusion protein containing N-terminal amino acids 301-407 (the bHLH/leucine zipper domain), was used as immunogen. The antibody recognizes both the 125 kDa precursor and 60-70 kDa mature, cleaved forms of SREBP-1.