The chicken liver cation-independent mannose 6-phosphate receptor lacks the high affinity binding site for insulin-like growth factor II

The chicken liver cation-independent mannose 6-phosphate receptor has been purified to apparent homogeneity by affinity chromatography on pentamannose phosphate-Sepharose and tested for its ability to bind iodinated human IGF-I, human IGF-II, and chicken IGF-II. In contrast to the bovine, rat, and human cation-independent mannose 6-phosphate receptors, which bind human IGF-II and IGF-I with nanomolar and micromolar affinities, respectively, the chicken receptor failed to bind either radioligand at receptor concentrations as high as 1 μM. The bovine receptor binds chicken IGF-II with high affinity while the chicken receptor binds this ligand with only low affinity, which we estimate to be in the micromolar range. These data demonstrate that the chicken cation-independent mannose 6-phosphate receptor lacks the high affinity binding site for IGF-II. These results provide an explanation for the failure of previous investigators to identify the type II IGF receptor by IGF-II cross-linking to chicken cells and indicate that the mitogenic activity of IGF-II in chick embryo fibroblasts is most likely mediated via the type I IGF receptor.