The two mannose 6-phosphate (M6P) receptors were identified because of their ability to bind M6P-containing soluble acid hydrolases in the Golgi and transport them to the endosomal-lysosomal system. During the past decade, we have started to understand the structural features of these receptors that allow them to do this job, and how the receptors themselves are sorted as they pass through various membrane-bound compartments. But trafficking of acid hydrolases is only part of the story. Evidence is emerging that one of the receptors can regulate cell growth and motility, and that it functions as a tumour suppressor.

No abstract

Targeting of newly synthesized lysosomal hydrolases to the lysosome is mediated by the cation-dependent mannose 6-phosphate receptor (CD-MPR) and the insulin-like growth factor II/cation-independent mannose 6-phosphate receptor (IGF-II/CI-MPR). The two receptors, which share sequence similarities, constitute the P-type family of animal lectins. We now report the three-dimensional structure of a glycosylation-deficient, yet fully functional form of the extracytoplasmic domain of the bovine CD-MPR (residues 3-154) complexed with mannose 6-phosphate at 1.8 A resolution. The extracytoplasmic domain of the CD-MPR crystallizes as a dimer, and each monomer folds into a nine-stranded flattened beta barrel, which bears a striking resemblance to avidin. The distance of 40 A between the two ligand-binding sites of the dimer provides a structural basis for the observed differences in binding affinity exhibited by the CD-MPR toward various lysosomal enzymes.

The cation-independent mannose 6-phosphate/insulin-like growth factor II receptor (M6P/IGF-II receptor) undergoes constitutive endocytosis, mediating the internalization of two unrelated classes of ligands, mannose 6-phosphate (Man-6-P)-containing acid hydrolases and insulin-like growth factor II (IGF-II). To determine the role of ligand valency in M6P/IGF-II receptor-mediated endocytosis, we measured the internalization rates of two ligands, ␤-glucuronidase (a homotetramer bearing multiple Man-6-P moieties) and IGF-II. We found that ␤-glucuronidase entered the cell ϳ3-4-fold faster than IGF-II. Unlabeled ␤-glucuronidase stimulated the rate of internalization of 125 I-IGF-II to equal that of 125 I-␤-glucuronidase, but a bivalent synthetic tripeptide capable of occupying both Man-6-P-binding sites on the M6P/IGF-II receptor simultaneously did not. A mutant receptor with one of the two Man-6-P-binding sites inactivated retained the ability to internalize ␤-glucuronidase faster than IGF-II. Thus, the increased rate of internalization required a multivalent ligand and a single Man-6-P-binding site on the receptor. M6P/IGF-II receptor solubilized and purified in Triton X-100 was present as a monomer, but association with ␤-glucuronidase generated a complex composed of two receptors and one ␤-glucuronidase. Neither IGF-II nor the synthetic peptide induced receptor dimerization. These results indicate that intermolecular cross-linking of the M6P/IGF-II receptor occurs upon binding of a multivalent ligand, resulting in an increased rate of internalization.The mannose 6-phosphate/insulin-like growth factor II receptor (M6P/IGF-II receptor) 1 is a type I transmembrane glycoprotein that cycles through the Golgi, endosomes, and the plasma membrane to carry out its role in the biogenesis of lysosomes and in the clearance of the polypeptide insulin-like growth factor II (IGF-II) (1, 2). In the Golgi, the receptor binds newly synthesized acid hydrolases modified with mannose 6-phosphate (Man-6-P) residues on their asparagine-linked oligosaccharides and transports them to endosomes via clathrincoated vesicles (3-5). The acid hydrolases are released in the acidified endosome and then packaged into lysosomes while the receptor either returns to the Golgi to bind another ligand or moves to the plasma membrane (6, 7). At the plasma membrane, the M6P/IGF-II receptor mediates internalization of Man-6-P-containing ligands and IGF-II (3,5,8).The interactions of IGF-II and Man-6-P-containing ligands with the M6P/IGF-II receptor have been characterized in several studies (8 -12). The extracellular portion of the M6P/ IGF-II receptor contains 15 homologous repeating domains of ϳ147 amino acids each (13). Domains 3 and 9 (numbering from the amino terminus) each bind 1 mol of Man-6-P, and the single IGF-II-binding site has been mapped to domain 11 in the extracellular region (14 -16). Man-6-P residues do not inhibit binding of IGF-II to the receptor, verifying that the two ligandbinding sites are distinct. However, proteins containing Man-6...

Background: Acid α-glucosidase, an enzyme replacement therapy for Pompe disease, is poorly targeted to lysosomes when relying on phosphomannose residues.Results: Fusing IGF-II to acid α-glucosidase resulted in more efficient uptake and glycogen clearance from muscle of Pompe mice.Conclusion: Enhanced binding to the cation-independent mannose 6-phosphate receptor (CI-MPR) enabled improved glycogen clearance in Pompe mice.Significance: BMN 701 is now being tested for Pompe disease in human clinical studies.