SCAP
Domain
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.
Function
Escort protein required for cholesterol as well as lipid homeostasis (By similarity). 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 (PubMed:26311497). 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 (By similarity). 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 (PubMed:26311497). Binds cholesterol via its SSD domain (By similarity).
Post-translational modifications
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.
Sequence Similarities
Belongs to the WD repeat SCAP family.
Cellular localization
- Endoplasmic reticulum membrane
- Multi-pass membrane protein
- Golgi apparatus membrane
- Multi-pass membrane protein
- Cytoplasmic vesicle
- COPII-coated vesicle membrane
- Multi-pass membrane protein
- Moves from the endoplasmic reticulum to the Golgi in the absence of sterols (PubMed:26311497). Requires the presence of SPRING1 for proper localization to endoplasmic reticulum (PubMed:32111832).
Alternative names
KIAA0199, PSEC0227, SCAP, Sterol regulatory element-binding protein cleavage-activating protein, SREBP cleavage-activating protein