Structural basis for sterol sensing by Scap and Insig

The sterol regulatory element-binding protein (SREBP) pathway senses the cellular cholesterol level through sterol regulated association between Scap and Insig. Despite the recent structural determination of the transmembrane domains of human Scap and Insig-2 bound to 25-hydroxycholesterol (25HC), the structure and regulatory mechanism of the luminal domains of Scap by cholesterol remains elusive. Here, combining cryo-EM analysis and artificial intelligence-facilitated structural prediction, we report the structure of the human Scap/Insig-2 complex in the presence of digitonin instead of 25HC. Despite the lack of sequence similarity, the structure of the luminal domain Loop 1 and a co-folded segment in Loop 7 of Scap resembles that of the luminal/extracellular domain in NPC1 and related proteins. Comparison of the sterol-loaded structures of these proteins provides clues of the regulation of Loop 1/7 interaction by cholesterol. We also show that the structure of Scap(D428A), which suppresses SREBP activation under sterol depletion, is identical to WT when complexed with Insig-2, although the gain of function may also involve a later step in protein trafficking.