CHARACTERIZATION OF MANNOSE 6-PHOSPHATE RECEPTORS (MPRS) FROM OPOSSUMLIVER - OPOSSUM CATION-INDEPENDENT MPR BINDS INSULIN-LIKE GROWTH FACTOR-II

Bovine, human, and rat cation-independent mannose 6-phosphate receptor s (CI-MPRs) are capable of binding both mannose 6-phosphate and insuli n-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 rel ated to placental mammals can bind IGF-II, the MPRs were purified from a marsupial, the American opossum (Didelphis virginiana), by phosphom annan-Sepharose affinity chromatography and then tested for their abil ity to bind IGF-11. Opossum liver expressed both the CI-MPR and the ca tion-dependent MPR (CD-MPR). Both receptors contained Asn-linked oligo saccharides. In contrast to CD-MPRs isolated from other species, the o possum CD-MPR displayed heterogeneity with respect to the number of As n-linked oligosaccharide chains it contains. The CI-MPR isolated from opossum liver, like the CI-MPR from bovine liver, bound iodinated huma n recombinant IGF-II. However, Scatchard analysis revealed that the op ossum CI-MPR bound IGF-11 with a lower affinity (K(d) = 14.5 nm) than the bovine receptor (K(d) = 0.2 nm). The addition of excess IGF-II, bu t not IGF-I or insulin, inhibited binding to [I-125]IGF-II, indicating that the opossum CI-MPR exhibits specificity for IGF-II. These result s 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.