Ala , unlike Dom9 Ala , was unable to completely dissociate from ligand under acidic pH conditions. These data indicate that the two Man-6-P binding sites of the IGF-II/MPR are not functionally equivalent.

Bovine, human, and rat cation-independent mannose 6-phosphate receptors (CI-MPRs) are capable of binding both mannose 6-phosphate and insulin-like growth factor-II (IGF-II). However, the receptor isolated from either chicken or frog lacks the high affinity IGF-II-binding site. To determine whether CI-MPRs isolated from a species that is closely related to placental mammals can bind IGF-II, the MPRs were purified from a marsupial, the American opossum (Didelphis virginiana), by phosphomannan-Sepharose affinity chromatography and then tested for their ability to bind IGF-II. Opossum liver expressed both the CI-MPR and the cation-dependent MPR (CD-MPR). Both receptors contained Asn-linked oligosaccharides. In contrast to CD-MPRs isolated from other species, the opossum CD-MPR displayed heterogeneity with respect to the number of Asn-linked oligosaccharide chains it contains. The CI-MPR isolated from opossum liver, like the CI-MPR from bovine liver, bound iodinated human recombinant IGF-II. However, Scatchard analysis revealed that the opossum CI-MPR bound IGF-II with a lower affinity (Kd = 14.5 nM) than the bovine receptor (Kd = 0.2 nM). The addition of excess IGF-II, but not IGF-I or insulin, inhibited binding to [125I]IGF-II, indicating that the opossum CI-MPR exhibits specificity for IGF-II. These results suggest that the emergence of a high affinity IGF-II-binding site in the CI-MPR occurred in evolution before the divergence of marsupials and placental mammals from their last common ancestor.

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