Insulin receptor
المؤلف:
Holt, Richard IG, and Allan Flyvbjerg
المصدر:
Textbook of diabetes (2024)
الجزء والصفحة:
6th ed , page112-114
2025-10-22
55
Extracellular domain of the insulin receptor
The insulin receptor proreceptor is a 150 kb gene comprising 22 exons on human chromosome 19p13.3–p13.2 that encodes the extracellular and intracellular domains (Figure 1a). During syn thesis, the proreceptor enters the endoplasmic reticulum where it is glycosylated and stabilized by disulfide bonds. Next it migrates into the Golgi, where furin cleaves at the Arg- Lys- Arg- Arg motif to generate the αβαβ- dimer (Figure 1a). The extracellular domain comprises the α- subunit and an NH2- terminal portion of the β- subunit. The α- subunit begins with two NH2- terminal leucine- rich repeats (L1, aa1–155; and L2, aa315–470) flanking a cysteine- rich (CR, aa195–310) region containing eight modular cysteine- linked repeats (Figure 1a). These modules are followed by three fibronectin- III motifs, including Fniii1 (aa473–591) and Fniii3 (aa808–904) flanking Fniii2 (aa597–717), which are interrupted by a 120 amino acid insert domain containing the furin cleavage site (IDa, aa636–690) (Figure 1a).
Fig1. (a) Diagram of the insulin receptor precursor showing the position of important domains, including two leucine- rich regions (L1 and L2) flanking a cysteine- rich region (CR); fibronectin iii domains (Fniii1, Fniii2a, Fniii2b, Fniii3); the insert domains IDa and IDb flanking the Arg- Lys- Arg- Arg furin cleavage site; the alternative IRA/IRB splice site that generates the short CTIRA or long CTIRB; a transmembrane domain (TMD); the IRS binding motif (NPEpY); the A- loop (activation loop) autophosphorylation sites; and carboxyl terminal tyrosine phosphorylation sites (CT). (b) Cartoon of the inactive conformation of the ectodomain of the insulin receptor (pdb id: 6pxv). The insulin receptor subunit is shown as a ribbon diagram with the labelled structural domains, L1’, CR’, L2’, Fniii1’, Fniii2a’, and CTa’ traced with a black line; Fniii2b’ and Fniii3’ are shown in light purple and grey space- filling structures, respectively. The other insulin receptor subunit is shown as a space- filling structure in the background: L1 (cyan), CR (yellow), L2 (orange), Fniii1 (green), Fniii2a (purple), and CTa (purple). Fniii2b’ and Fniii3’ are shown in light pink. (c) The structure of the ectodomain of the insulin receptor in the T conformation with insulin bound at S1 and S2. Note the different orientation of each insulin molecule revealed by the position of the B- chain residues FB1 and TB30. (d) The dimeric structure of the juxtamembrane domain (JMD) of the insulin receptor shown in green (IRK) and red (IRK’). The grey oval highlights the dimerization region between the juxtamembrane domain and the Ca/b- sheet. Phosphorylated residues of the A’- loop are shown in red (insulin) and green (insulin receptor).
The disulfide- linked α- subunits and extracellular portion of the β- subunits fold into an inverted V configuration with the apex formed by L2 and Fniii1 domains (Figure 9.2b). Natural and site- directed mutagenesis, photo- affinity labelling, and structural refinements predicted the location of two insulin binding sites in the α- subunit. The high- affinity S1 is composed of the L1 domain from one α- subunit and CTα′ (aa 693–710) from the other (Figure 1c). When the insulin receptor is saturated by high insulin concentrations, cryo- EM reveals the location of a distinct low- affinity S2 near the Fniii1 → Fniii2a interface (Figure 1c). Insulin binding to S1 crosslinks the α- subunits to stabilize the extracellular domain in a T conformation, which might cause the Fniii3 → transmembrane domain (TMD) to converge and facilitate transphosphorylation of the intracellular receptor tyrosine kinase (Figure 1d).
Exon- 11 of the insulin receptor gene is alternatively spliced depending on the tissue and developmental stage to produce two insulin receptor isoforms, including IRA that omits exon- 11 and IRB that includes exon- 11 to add 12 amino acids at the C- terminus of the α- subunit (CTα) [15]. Insulin binds with high affinity to the homodimeric IRB (αβIR•αβIR), which predominates in adult liver, muscle, and adipose tissues. IRA binds insulin with slightly lower affinity, but unlike IRB also binds IGF- I and IGF- II in the physio logical range.
Receptor tyrosine kinase
Most receptor tyrosine kinases reside in the plasma membrane as monomers that form homo- or heterodimers on ligand binding to promote transphosphorylation and signal transduction. Since the insulin receptor is a covalent dimer, a different mechanism inhibits transphosphorylation of the cytoplasmic receptor tyrosine kinase until insulin binds. The intracellular portion of the insulin receptor β- subunit begins with a 30- residue juxtamembrane domain between the TMD and the receptor tyrosine kinase that terminates with the 70- residue carboxy- terminus (Figure 1a). Each intracellular region contains tyrosyl phosphorylation sites (numbered as in IRB): Y965 and Y972 in the juxtamembrane domain; Y1158, Y1162, and Y1163 in the activation loop (A- loop) of the kinase domain; and Y1328 and Y1334 in the carboxy- terminus. Recent analysis using liquid chromatography–tandem mass spectrometry revealed several serine phosphorylation sites, including S968, S969, S974, and S976 in the juxtamembrane domain; and S1278, S1320, S1321, and T1348 in the carboxy- terminus.
Before insulin binds, the catalytic activity of the insulin receptor is inhibited by interactions between Tyr984 in the juxtamembrane domain with a hydrophobic pocket created from an α- helix (αC) and a five- stranded β- sheet in the N- terminal lobe of the receptor tyrosine kinase. This interaction inhibits transphosphorylation of the adjacent receptor tyrosine kinases by preventing the α- helix from assuming its catalytically active position. Both receptor tyrosine kinase domains are inhibited until the ectodomains converge during insulin binding, which is thought to facilitate transphosphorylation in the A- loop (Figure 1c). Multisite transphosphorylation in the A- loop releases several inhibitory mechanisms:
• The unphosphorylated Tyr1162, the second of the three A- loop tyrosine residues, blocks access of peptide substrates to the active site.
• Asp1161 preceding Tyr1162 stabilizes the closed A- loop to inhibit unstimulated transphosphorylation.
• The NH2- terminal end of the A- loop (D1150FG- motif) competes to elevate the Km for adenosine triphosphate (ATP).
Intramolecular transphosphorylation begins at Tyr1158 and progresses to Tyr1162 and slowly to Tyr1163 until tris- phosphorylation stabilizes the open A- loop to fully activate the kinase. This model is confirmed by an Asp1161 → Ala substitution that shifts the A- loop towards the open configuration, increasing basal activity. Substitution of Tyr1162 with phenylalanine also increases basal transphosphorylation consistent with its role to stabilize the closed conformation or block ATP and protein substrate entry into the active site. Transphosphorylation appears to destabilize the cis- interaction between Tyr984 and α- helix, creating a new trans- interaction between Tyr984 and αC′ or Tyr984′ and the α- helix of adjacent receptor tyrosine kinases (Figure 1d). Thus, kinase activation involves both activation- loop phosphorylation and con formation rearrangements that stabilize the receptor tyrosine kinase dimer, so that the catalytic sites are exposed on opposite sides of the insulin receptor kinase to phosphorylate IRS- proteins or SHC recruited by the NPEpY- motif (Figure 1d).
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