Overexpression of membrane domain of SCAP prevents sterols from inhibitingSCAP center dot SREBP exit from endoplasmic reticulum

SCAP (SREBP cleavage-activating protein) forms a complex with sterol regula tory element-binding proteins (SREBPs) and escorts them from the endoplasmi c reticulum (ER) to the Golgi complex where proteases release transcription ally active segments of SREBPs, which enter the nucleus to activate lipid s ynthesis. The NH2-terminal segment of SCAP contains eight transmembrane hel ices, five of which (TM2-6) comprise the sterol-sensing domain. This domain responds to sterols by causing the SCAP SREBP complex to be retained in th e ER, preventing proteolytic release and reducing transcription of lipogeni c genes. Here, we use transfection techniques to overexpress a segment of S CAP containing transmembrane helices 1-6 in hamster and human cells. This s egment does not interfere with SCAP SREBP movement to the Golgi in the abse nce of sterols, but it prevents sterols from suppressing this movement. Thi s block is abolished when SCAP(TM1-6) contains a point mutation (Y298C) tha t is known to abolish the activity of the sterol-sensing domain. We interpr et these findings to indicate that sterols cause the SCAP SREBP complex to bind to an ER retention protein through an interaction that involves the st erol-sensing domain. The SCAP(TM1-6) segment competes with the SCAP SREBP c omplex for binding to this putative retention protein, thereby liberating t he SCAP SREBP complex so that it can move to the Golgi despite the presence of sterols, These studies provide a potential mechanistic explanation for the ability of sterols to block SCAP SREBP movement from the ER and thereby to control lipid synthesis in animal cells.