PURIFICATION AND CDNA CLONING OF A 2ND APOPTOSIS-RELATED CYSTEINE PROTEASE THAT CLEAVES AND ACTIVATES STEROL REGULATORY ELEMENT-BINDING PROTEINS

We have purified from hamster liver a second cysteine protease that cl eaves and activates sterol regulatory element binding proteins (SREBPs ). cDNA cloning revealed that this enzyme is the hamster equivalent of Mch3, a human enzyme that is related to the interleukin 1 beta conver ting enzyme. We call this enzyme Mch3/SCA-2. It is 54% identical to ha mster CPP32/SCA-1, a cysteine protease that was earlier shown to cleav e SREBPs at a conserved Asp between the basic helix-loop-helix leucine zipper domain and the membrane attachment domain. This cleavage liber ates an NH2-terminal fragment of approximate to 460 amino acids that a ctivates transcription of genes encoding the low density lipoprotein r eceptor and enzymes of cholesterol synthesis. Mch3/SCA-2 and CPP32/SCA -1 are synthesized as inactive 30-35 kDa precursors that are thought t o be cleaved during apoptosis to generate active fragments of approxim ate to 20 and approximate to 10 kDa. The current data lend further sup port to the notion that SREBPs are cleaved and activated as part of th e program in programmed cell death.