Sterols regulate cycling of SREBP cleavage-activating protein (SCAP) between endoplasmic reticulum and Golgi

The proteolytic cleavage of sterol regulatory element-binding proteins (SRE BPs) is regulated by SREBP cleavage-activating protein (SCAP), which forms complexes with SREBPs in membranes of the endoplasmic reticulum (ER). In st erol-depleted cells, SCAP facilitates cleavage of SREBPs by Site-1 protease , thereby initiating release of active NH2-terminal fragments from the ER m embrane so that they can enter the nucleus and activate gene expression. In sterol-overloaded cells, the activity of SCAP is blocked, SREBPs remain bo und to membranes, and transcription of sterol-regulated genes declines. Her e, we provide evidence that sterols act by inhibiting the cycling of SCAP b etween the ER and Golgi. We use glycosidases, glycosidase inhibitors, and a glycosylation-defective mutant cell line to demonstrate that the N-linked carbohydrates of SCAP are modified by Golgi enzymes in sterol-depleted cell s. After modification, SCAP returns to the ER, as indicated by experiments that Show that the Golgi-modified forms of SCAP cofractionate with ER membr anes on density gradients. In sterol-overloaded cells, the Golgi modificati ons of SCAP do not occur, apparently because SCAP fails to leave the ER. Go lgi modifications of SCAP are restored when sterol overloaded cells are tre ated with brefeldin A,which causes Golgi enzymes to translocate to the ER. These studies suggest that sterols regulate the cleavage of SREBPs by modul ating the ability of SCAP to transport SREBPs to a post-ER compartment that houses active Site-1 protease.