9.B.418. The Insulin-induced Gene 1 & 2 Protein (INSIG) Family
INSIG 1 and 2 are oxysterol-binding proteins that mediates feedback
control of cholesterol synthesis by controlling both endoplasmic
reticulum to Golgi transport of SCAP and degradation of HMGCR (Yabe et al. 2002). It acts as a negative regulator of cholesterol biosynthesis by mediating
the retention of the SCAP-SREBP complex in the endoplasmic reticulum,
thereby blocking the processing of sterol regulatory element-binding
proteins (SREBPs) SREBF1/SREBP1 and SREBF2/SREBP2 (Xu et al. 2020). It binds oxysterols, including 22-hydroxycholesterol,
24-hydroxycholesterol, 25-hydroxycholesterol and 27-hydroxycholesterol,
regulating interaction with SCAP and retention of the SCAP-SREBP complex
in the endoplasmic reticulum (Radhakrishnan et al. 2007).
In the presence of oxysterol, it interacts with SCAP, retaining the SCAP-SREBP
complex in the endoplasmic reticulum, thereby preventing SCAP from
escorting SREBF1/SREBP1 and SREBF2/SREBP2 to the Golgi (PubMed:32322062).
Sterol deprivation or phosphorylation by PCK1 reduces oxysterol-binding,
disrupting the interaction between INSIG2 and SCAP, thereby promoting
Golgi transport of the SCAP-SREBP complex, followed by processing and
nuclear translocation of SREBF1/SREBP1 and SREBF2/SREBP2 (Xu et al. 2020). It also regulates cholesterol synthesis by regulating the degradation of
HMGCR: it initiates the sterol-mediated ubiquitin-mediated ER-associated degradation (ERAD) of HMGCR via recruitment of the
reductase to the ubiquitin ligase RNF139 (Gong et al. 2006, Jo et al. 2011).
The cryo-EM structure of human SCAP bound to Insig-2 suggests how their interaction is regulated by sterols (Yan et al. 2021). The sterol regulatory element-binding protein (SREBP) pathway controls cellular homeostasis of sterols. The key players in this pathway, Scap and Insig-1 and -2, are membrane-embedded sterol sensors. The 25-hydroxycholesterol (25HC)-dependent association of Scap and Insig acts as the master switch for the SREBP pathway. Yan et al. 2021 presented cryo-EM analyses of the human Scap and Insig-2 complex in the presence of 25HC, with the transmembrane (TM) domains determined at an average resolution of 3.7 Å. The sterol-sensing domain in Scap and all six TMSs in Insig-2 were resolved. A 25HC molecule is sandwiched between the S4 to S6 segments in Scap and TMSs 3 and 4 in Insig-2 in the luminal leaflet of the membrane. Unwinding of the middle of the Scap-S4 segment is crucial for 25HC binding and Insig association (Yan et al. 2021).