2.A.6.6.4
SREBP cleavage-activating protein, Scap of 1279 aas.  Cholesterol homeostasis is mediated by Scap, a polytopic ER protein that transports SREBPs from ER to Golgi where SREBPs are processed to forms that activate cholesterol synthesis. Scap has eight transmembrane helices and two large luminal loops, designated Loop1 and Loop7. Evidence suggests that Loop1 binds to Loop7, allowing Scap to bind COPII proteins for transport in coated vesicles (Zhang et al. 2016). When ER cholesterol rises, it binds to Loop1 causeing dissociation from Loop7, abrogating COPII binding. Direct binding of the two loops causes dissociation from the membrane, allowing the soluble complex to be secreted.  Point mutations that disrupt the Loop1-Loop7 interaction prevented secretion. The cryo-EM structure of human SCAP bound to Insig-2 suggests how their interaction is regulated by sterols (Yan et al. 2021). The sterol regulatory element-binding protein (SREBP) pathway controls cellular homeostasis of sterols. The key players in this pathway, Scap and Insig-1 and -2, are membrane-embedded sterol sensors. The 25-hydroxycholesterol (25HC)-dependent association of Scap and Insig acts as the master switch for the SREBP pathway. Yan et al. 2021 presented cryo-EM analyses of the human Scap and Insig-2 complex in the presence of 25HC, with the transmembrane (TM) domains determined at an average resolution of 3.7 Å. The sterol-sensing domain in Scap and all six TMSs in Insig-2 were resolved. A 25HC molecule is sandwiched between the S4 to S6 segments in Scap and TMSs 3 and 4 in Insig-2 in the luminal leaflet of the membrane. Unwinding of the middle of the Scap-S4 segment is crucial for 25HC binding and Insig association (Yan et al. 2021).