Crucial step in cholesterol homeostasis: Sterols promote binding of SCAP to INSIG-1, a membrane protein that facilitates retention of SREBPs in ER

Using coimmunoprecipitation and tandem mass spectrometry, we identify INSIG-1 as an ER protein that binds the sterol-sensing domain of SREBP cleavage-activating protein (SCAP) and facilitates retention of the SCAP/SREBP complex in the ER. In steroldepleted cells, SCAP escorts SREBPs from ER to Golgi for proteolytic processing, thereby allowing SREBPs to stimulate cholesterol synthesis. Sterols induce binding of SCAP to INSIG-11, as determined by blue native-PAGE, and this is correlated with the inhibition of SCAP exit from the ER. Overexpression of INSIG-1 increases the sensitivity of cells to sterol-mediated inhibition of SREBP processing. Mutant SCAP(Y298C) fails to bind INSIG-11 and is resistant to sterol-mediated inhibition of ER exit. By facilitating sterol -dependent ER retention of SCAP, INSIG-11 plays a central role in cholesterol homeostasis.