TOPOLOGY OF SREBP CLEAVAGE-ACTIVATING PROTEIN, A POLYTOPIC MEMBRANE-PROTEIN WITH A STEROL-SENSING DOMAIN

The NH2-terminal fragments of sterol regulatory element-binding protei ns (SREBPs) are released from endoplasmic reticulum membranes by prote ases whose activities depend upon SREBP cleavage-activating protein (S CAP), a polytopic endoplasmic reticulum membrane protein. The activity of SCAP is inhibited by sterols, which appear to interact with the po lytopic membrane domain of SCAP. Here, we use protease protection and N-linked glycosylation site-mapping techniques to define the topology of the eight membrane-spanning domains of SCAP, The data indicate that the NH,terminus and COOH terminus of SCAP face the cytosol, The long intralumenal loops after membrane-spanning segments I and 7 are glycos ylated, confirming their lumenal location. The region comprising membr ane-spanning segments 2-6 shows sequence resemblance to putative stero l-sensing domains in three other proteins: 3-hydroxy-3-methylglutaryl CoA reductase (HMG-CoA reductase), the Niemann-Pick CI protein, and th e morphogen receptor Patched. The orientation of the eight membrane-sp anning segments in SCAP is consistent with the model proposed for HMG- CoA reductase (Olender, E, H,, and Simoni, R. D, (1992) J. Biol. Chem. 267, 4223-4235), The membrane-spanning domains of SCAP and HMG-CoA re ductase confer sterol sensitivity upon the functional activities of th e two molecules. The common membrane topology of the two proteins is c onsistent with the notion that sterols regulate both proteins by a com mon mechanism.