Structural basis for sterol sensing by Scap and Insig

Cell Rep. 2021 Jun 29;35(13):109299. doi: 10.1016/j.celrep.2021.109299.

Abstract

The sterol regulatory element-binding protein (SREBP) pathway monitors the cellular cholesterol level through sterol-regulated association between the SREBP cleavage-activating protein (Scap) and the insulin-induced gene (Insig). Despite structural determination of the Scap and Insig-2 complex bound to 25-hydroxycholesterol, the luminal domains of Scap remain unresolved. In this study, combining cryogenic electron microscopy (cryo-EM) analysis and artificial intelligence-facilitated structural prediction, we report the structure of the human Scap/Insig-2 complex purified in digitonin. The luminal domain loop 1 and a co-folded segment in loop 7 of Scap resemble those of the luminal/extracellular domain in NPC1 and related proteins, providing clues to the cholesterol-regulated interaction of loop 1 and loop 7. An additional luminal interface is observed between Scap and Insig. We also show that Scap(D428A), which inhibits SREBP activation even under sterol depletion, exhibits an identical conformation with the wild-type protein when complexed with Insig-2, and its constitutive suppression of the SREBP pathway may also involve a later step in protein trafficking.

Keywords: Insig; Loop 1; Loop 7; SREBP pathway; Scap; cholesterol metabolism; cryo-EM structure; sterol sensing domain.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Digitonin / chemistry
  • HEK293 Cells
  • Humans
  • Intracellular Signaling Peptides and Proteins / chemistry*
  • Intracellular Signaling Peptides and Proteins / metabolism*
  • Membrane Proteins / chemistry*
  • Membrane Proteins / metabolism*
  • Micelles
  • Models, Molecular
  • Protein Conformation
  • Protein Folding
  • Sterols / chemistry*
  • Sterols / metabolism*
  • Structural Homology, Protein

Substances

  • INSIG2 protein, human
  • Intracellular Signaling Peptides and Proteins
  • Membrane Proteins
  • Micelles
  • SREBP cleavage-activating protein
  • Sterols
  • Digitonin