The Two Mannose 6-Phosphate Binding Sites of the Insulin-like Growth Factor-II/Mannose 6-Phosphate Receptor Display Different Ligand Binding Properties*

The two mannose 6-phosphate (Man-6-P) binding sites of the insulin-like growth factor-II/mannose 6-phosphate receptor (IGF-II/MPR) have been localized to domains 1–3 and 7–9, and studies have shown that Arg435 in domain 3 and Arg1334 in domain 9 are essential for Man-6-P binding. To determine whether the IGF-II/MPR containing a single Man-6-P binding site is functional, clonal mouse L cell lines stably transfected with either mutant bovine IGF-II/MPR cDNA, containing substitutions at position 435 and/or 1334, or the wild type receptor cDNA were assayed for their ability to sort lysosomal enzymes to the lysosome. Mutant receptors containing a single Man-6-P binding site were ∼50% less efficient than the wild type receptor in the overall targeting of lysosomal enzymes to the lysosome. Mutant receptors containing a substitution at Arg1334 (Dom9Ala), in contrast to those containing a substitution at Arg435(Dom3Ala), were unable to target cathepsin D and β-hexosaminidase to the lysosome. Equilibrium binding assays using125I-labeled β-glucuronidase demonstrated that Dom3Ala and Dom9Ala had aK d of 2.0 and 4.3 nm, respectively. In addition, Dom3Ala, unlike Dom9Ala, 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.