Autocatalytic processing of site-1 protease removes propeptide and permits cleavage of sterol regulatory element-binding proteins

Site-1 protease (S1P) is a subtilisin-related protease that cleaves sterol regulatory element-binding proteins (SREBPs) in the endoplasmic reticulum lumen, thereby initiating a process by which the transcriptionally active NH₂-terminal fragments of SREBPs are released from membranes. In the current experiments, we transfected cDNAs encoding epitope-tagged hamster S1P into HEK-293 cells or mutant hamster cells that lack S1P. Protease protection assays showed that the bulk of S1P is in the endoplasmic reticulum lumen, anchored by a COOH-terminal membrane-spanning segment. Cleavage of the NH₂- terminal signal sequence of S1P generates S1P-A (amino acids 23-1052), which is inactive. The protein is self-activated by an intramolecular cleavage at Site-B, generating S1P-B (amino acids 138-1052) and liberating a 115-amino acid propeptide that is secreted intact into the medium. The sequence at Site-B is RSLK, which differs from the RSVL sequence at the cleavage site in SREBP-2. S1P-B is further cleaved at an internal RRLL sequence to yield S1P- C (amino acids 187-1052). Mutational analysis suggests that S1P-B and S1P-C are both active in cleaving SREBP-2 in a fashion that requires SREBP cleavage-activating protein. The activity of S1P-C may be short-lived because it appears to be transported to the Golgi, a site at which SREBP-2 cleavage may not normally occur. These data provide the initial description of the processing of a subtilisin-related protease that controls the level of cholesterol in blood and cells. In an accompanying paper (Cheng, D., Espenshade, P. J., Slaughter, C. A., Jaen, J. C., Brown, M. S., and Goldstein, J. L. (1999), J. Biol. Chem., 274, 22805-22812), we develop an in vitro assay to characterize the activity of purified recombinant S1P.