The structure of the IGF2R is a type I transmembrane protein (that is, it has a single transmembrane domain with its C-terminus on the cytoplasmic side of lipid membranes) with a large extracellular/lumenal domain and a relatively short cytoplasmic tail.[7] The extracellular domain consists of a small region homologous to the collagen-binding domain of fibronectin and of fifteen repeats of approximately 147 amino acid residues. Each of these repeats is homologous to the 157-residue extracytoplasmic domain of the mannose 6-phosphate receptor. Binding to IGF2 is mediated through one of the repeats, while two different repeats are responsible for binding to mannose-6-phosphate. The IGF2R is approximately 300 kDa in size; it appears to exist and function as a dimer.
Function
IGF2R functions to clear IGF2 from the cell surface to attenuate signalling, and to transport lysosomal acid hydrolase precursors from the Golgi apparatus to the lysosome. After binding IGF2 at the cell surface, IGF2Rs accumulate in forming clathrin-coated vesicles and are internalized. In the lumen of the trans-Golgi network, the IGF2R binds M6P-tagged cargo.[7] The IGF2Rs (bound to their cargo) are recognized by the GGA family of clathrin adaptor proteins and accumulate in forming clathrin-coated vesicles.[8] IGF2Rs from both the cell surface and the Golgi are trafficked to the early endosome where, in the relatively low pH environment of the endosome, the IGF2Rs release their cargo. The IGF2Rs are recycled back to the Golgi by the retromer complex, again by way of interaction with GGAs and vesicles. The cargo proteins are then trafficked to the lysosome via the late endosome independently of the IGF2Rs.
The insulin-like growth factor 2 receptor function evolved from the cation-independent mannose 6-phosphate receptor and is first seen in Monotremes. The IGF-2 binding site was likely acquired fortuitously with the generation of an exonic splice site enhancer cluster in exon 34, presumably necessitated by several kilobases of repeat element insertions in the preceding intron. A six-fold affinity maturation then followed during therian evolution, coincident with the onset of imprinting and consistent with the theory of parental conflict.[11]
^ abLaureys G, Barton DE, Ullrich A, Francke U (October 1988). "Chromosomal mapping of the gene for the type II insulin-like growth factor receptor/cation-independent mannose 6-phosphate receptor in man and mouse". Genomics. 3 (3): 224–9. doi:10.1016/0888-7543(88)90083-3. PMID2852162.
^ abGhosh P, Dahms NM, Kornfeld S (March 2003). "Mannose 6-phosphate receptors: new twists in the tale". Nat. Rev. Mol. Cell Biol. 4 (3): 202–12. doi:10.1038/nrm1050. PMID12612639. S2CID16991464.
^Ghosh P, Kornfeld S (July 2004). "The GGA proteins: key players in protein sorting at the trans-Golgi network". Eur. J. Cell Biol. 83 (6): 257–62. doi:10.1078/0171-9335-00374. PMID15511083.
Hawkes C, Kar S (2004). "The insulin-like growth factor-II/mannose-6-phosphate receptor: structure, distribution and function in the central nervous system". Brain Res. Brain Res. Rev. 44 (2–3): 117–40. doi:10.1016/j.brainresrev.2003.11.002. PMID15003389. S2CID20434586.
Scott CD, Firth SM (2005). "The role of the M6P/IGF-II receptor in cancer: tumor suppression or garbage disposal?". Horm. Metab. Res. 36 (5): 261–71. doi:10.1055/s-2004-814477. PMID15156403. S2CID260166901.
Antoniades HN, Galanopoulos T, Neville-Golden J, Maxwell M (1992). "Expression of insulin-like growth factors I and II and their receptor mRNAs in primary human astrocytomas and meningiomas; in vivo studies using in situ hybridization and immunocytochemistry". Int. J. Cancer. 50 (2): 215–22. doi:10.1002/ijc.2910500210. PMID1370435. S2CID22632952.
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De Souza AT, Hankins GR, Washington MK, Orton TC, Jirtle RL (1996). "M6P/IGF2R gene is mutated in human hepatocellular carcinomas with loss of heterozygosity". Nat. Genet. 11 (4): 447–9. doi:10.1038/ng1295-447. PMID7493029. S2CID21787312.
De Souza AT, Hankins GR, Washington MK, Fine RL, Orton TC, Jirtle RL (1995). "Frequent loss of heterozygosity on 6q at the mannose 6-phosphate/insulin-like growth factor II receptor locus in human hepatocellular tumors". Oncogene. 10 (9): 1725–9. PMID7753549.
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Ishiwata T, Bergmann U, Kornmann M, Lopez M, Beger HG, Korc M (1997). "Altered expression of insulin-like growth factor II receptor in human pancreatic cancer". Pancreas. 15 (4): 367–73. doi:10.1097/00006676-199711000-00006. PMID9361090. S2CID42073680.
