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).


 

References:

Gong, Y., J.N. Lee, M.S. Brown, J.L. Goldstein, and J. Ye. (2006). Juxtamembranous aspartic acid in Insig-1 and Insig-2 is required for cholesterol homeostasis. Proc. Natl. Acad. Sci. USA 103: 6154-6159.
Jo, Y., P.C. Lee, P.V. Sguigna, and R.A. DeBose-Boyd. (2011). Sterol-induced degradation of HMG CoA reductase depends on interplay of two Insigs and two ubiquitin ligases, gp78 and Trc8. Proc. Natl. Acad. Sci. USA 108: 20503-20508.
Radhakrishnan, A., Y. Ikeda, H.J. Kwon, M.S. Brown, and J.L. Goldstein. (2007). Sterol-regulated transport of SREBPs from endoplasmic reticulum to Golgi: oxysterols block transport by binding to Insig. Proc. Natl. Acad. Sci. USA 104: 6511-6518.
Wang, Y.J., Y. Bian, J. Luo, M. Lu, Y. Xiong, S.Y. Guo, H.Y. Yin, X. Lin, Q. Li, C.C.Y. Chang, T.Y. Chang, B.L. Li, and B.L. Song. (2017). Cholesterol and fatty acids regulate cysteine ubiquitylation of ACAT2 through competitive oxidation. Nat. Cell Biol. 19: 808-819.
Xu, D., Z. Wang, Y. Xia, F. Shao, W. Xia, Y. Wei, X. Li, X. Qian, J.H. Lee, L. Du, Y. Zheng, G. Lv, J.S. Leu, H. Wang, D. Xing, T. Liang, M.C. Hung, and Z. Lu. (2020). The gluconeogenic enzyme PCK1 phosphorylates INSIG1/2 for lipogenesis. Nature 580: 530-535.
Yabe, D., M.S. Brown, and J.L. Goldstein. (2002). Insig-2, a second endoplasmic reticulum protein that binds SCAP and blocks export of sterol regulatory element-binding proteins. Proc. Natl. Acad. Sci. USA 99: 12753-12758.
Yan, R., P. Cao, W. Song, H. Qian, X. Du, H.W. Coates, X. Zhao, Y. Li, S. Gao, X. Gong, X. Liu, J. Sui, J. Lei, H. Yang, A.J. Brown, Q. Zhou, C. Yan, and N. Yan. (2021). A structure of human Scap bound to Insig-2 suggests how their interaction is regulated by sterols. Science 371:.
Examples:

TC#NameOrganismal TypeExample
9.B.418.1.1

Insulin-induced gene 2 protein, INSIG2, of 225 aas and 6 TMSs. It functions with SCAP (TC# 2.A.6.6.4) to control the  of enzymes and transport systems involved in the biosynthesis of cholesterol and other sterols (Yan et al. 2021). See family description for details.

Insig-2 of Homo sapiens

 
9.B.418.1.2

Insulin-induced gene 1 protein, INSIG1, of 277 aas and 6 or 7 TMSs. Like INSIG2, it binds oxysterols, including 25-hydroxycholesterol, regulating interaction with SCAP and retention of the SCAP-SREBP complex in the ER (Xu et al. 2020). It also regulates degradation of SOAT2/ACAT2 when the lipid levels are low, and initiates the ubiquitin-mediated degradation of sterol O-acyltransferase 2 (SOAT2/ACAT2) (TC# 2.A.50.4.11) via recruitment of the ubiquitin ligases AMFR/gp78 (Wang et al. 2017). It resembles INSIG2 in many functional respects (see 9.B.418.1.1).

Insig-1 of Homo sapiens

 
9.B.418.1.3

Insulin-induced protein of 473 aas and 6 TMSs in a 1 + 1 + 4 (C-terminal) TMSs. It shows good sequence similarity to Insig-1 and -2 only in the 4 TMS C-terminal domain.

Insulin-induced protein of Ascosphaera apis

 
9.B.418.1.4

Uncharacterized protein of 373 aas and 10 or 11 TMSs in a 4 or 5 + 6 TMS arrangement.  The last 6 TMSs are members of this family.

UP of Planctomycetes bacterium

 
9.B.418.1.5

Uncharacterized protein of 185 aas and 6 TMSs.

UP of Hyalangium minutum

 
9.B.418.1.6

Uncharacterized protein of 213 aas and 6 TMSs.

UP of Mycobacteroides salmoniphilum

 
9.B.418.1.7

Multidomain LON peptidase N-terminal domain and RING finger protein 1of 848 aas and about 7 TMSs in a 1 + 1 + 3 + 1 + 1 TMS arrangement.  TMSs 2 - 6 are homologous to other members of this family.

Multidomain protein of Ophiocordyceps camponoti-floridani