Escort protein required for cholesterol as well as lipid homeostasis. Regulates export of the SCAP-SREBP complex from the endoplasmic reticulum to the Golgi upon low cholesterol, thereby regulating the processing of sterol regulatory element-binding proteins (SREBPs) SREBF1/SREBP1 and SREBF2/SREBP2. At high sterol concentrations, formation of a ternary complex with INSIG (INSIG1 or INSIG2) leads to mask the ER export signal in SCAP, promoting retention of the complex in the endoplasmic reticulum. Low sterol concentrations trigger release of INSIG, a conformational change in the SSD domain of SCAP, unmasking of the ER export signal, promoting recruitment into COPII-coated vesicles and transport of the SCAP-SREBP to the Golgi: in the Golgi, SREBPs are then processed, releasing the transcription factor fragment of SREBPs from the membrane, its import into the nucleus and up-regulation of LDLR, INSIG1 and the mevalonate pathway. Binds cholesterol via its SSD domain. Membrane region forms a homotetramer (By similarity). Forms a stable complex with SREBF1/SREBP1 or SREBF2/SREBP2 through its C-terminal cytoplasmic domain (By similarity). Forms a ternary complex with INSIG1 or INSIG2 through its transmembrane domains at high sterol concentrations (By similarity). Interacts with the SEC23-SEC24 complex in a SAR1-GTP-dependent manner through an ER export signal in its third cytoplasmic loop (By similarity). Binds cholesterol through its SSD domain (By similarity). Component of SCAP-SREBP complex composed of SREBF2, SCAP and RNF139; the complex hampers the interaction between SCAP and SEC24B, thereby reducing SREBF2 proteolytic processing (By similarity). Interacts with RNF139; the interaction inhibits the interaction of SCAP with SEC24B and hampering the ER to Golgi transport of the SCAP-SREBP complex (By similarity). Interacts with SPRING (By similarity). Moves from the endoplasmic reticulum to the Golgi in the absence of sterols. Requires the presence of SPRING for proper localization to endoplasmic reticulum. Loop-1 binds to loop-7, enabling interaction with COPII-coated vesicles. When levels of cholesterol in the endoplasmic reticulum increase, Loop-1 binds to cholesterol instead, thereby disrupting direct binding between the two loops and preventing the SCAP-SREBP complex from exiting the endoplasmic reticulum. Cholesterol bound to SSD domain of SCAP or oxysterol bound to INSIG (INSIG1 or INSIG2) leads to masking of an ER export signal (also named MELADL motif) on SCAP possibly by moving the signal further away from the ER membrane. Ubiquitinated at Lys-454 and Lys-466. RNF145 triggers ubiquitination of SCAP, likely inhibiting SCAP-SREBP complex transport to the Golgi apparatus and the subsequent processing/maturation of SREBF2/SREBP2. Belongs to the WD repeat SCAP family.