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High‐affinity ligand binding by wild‐type/mutant heteromeric complexes of the mannose
6‐phosphate/insulin‐like growth factor II receptor
Author(s) -
Hartman Michelle A.,
Kreiling Jodi L.,
Byrd James C.,
MacDonald Richard G.
Publication year - 2009
Publication title -
the febs journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.981
H-Index - 204
eISSN - 1742-4658
pISSN - 1742-464X
DOI - 10.1111/j.1742-4658.2009.06917.x
Subject(s) - mannose 6 phosphate , receptor , internalization , binding site , mannose , ligand (biochemistry) , mutant , biochemistry , insulin like growth factor 2 receptor , chemistry , biology , plasma protein binding , growth factor , insulin like growth factor 1 receptor , gene
The mannose 6‐phosphate/insulin‐like growth factor II receptor has diverse ligand‐binding properties contributing to its roles in lysosome biogenesis and growth suppression. Optimal receptor binding and internalization of mannose 6‐phosphate (Man‐6‐P)‐bearing ligands requires a dimeric structure leading to bivalent high‐affinity binding, presumably mediated by cooperation between sites on both subunits. Insulin‐like growth factor II (IGF‐II) binds to a single site on each monomer. It is hypothesized that IGF‐II binding to cognate sites on each monomer occurs independently, but bivalent Man‐6‐P ligand binding requires cooperative contributions from sites on both monomers. To test this hypothesis, we co‐immunoprecipitated differentially epitope‐tagged soluble mini‐receptors and assessed ligand binding. Pairing of wild‐type and point‐mutated IGF‐II binding sites between two dimerized mini‐receptors had no effect on the function of the contralateral binding site, indicating IGF‐II binding to each side of the dimer is independent and manifests no intersubunit effects. As expected, heterodimeric receptors composed of a wild‐type monomer and a mutant bearing two Man‐6‐P‐binding knockout mutations form functional IGF‐II binding sites. By contrast to prediction, such heterodimeric receptors also bind Man‐6‐P‐based ligands with high affinity, and the amount of binding can be attributed entirely to the immunoprecipitated wild‐type receptors. Anchoring of both C‐terminal ends of the heterodimer produces optimal binding of both IGF‐II and Man‐6‐P ligands. Thus, IGF‐II binds independently to both subunits of the dimeric mannose 6‐phosphate/insulin‐like growth factor II receptor. Although wild‐type/mutant hetero‐oligomers form readily when mixed, it appears that multivalent Man‐6‐P ligands bind preferentially to wild‐type sites, possibly by cross‐bridging receptors within clusters of immobilized receptors.