title: Twists and turns of the cation-dependent mannose 6-phosphate receptor. Ligand-bound versus ligand-free receptor.
abstract: Mannose 6-phosphate receptors (MPRs) participate in the biogenesis of lysosomes in higher eukaryotes by transporting soluble acid hydrolases from the trans-Golgi network to late endosomal compartments. The receptors release their ligands into the acidic environment of the late endosome and then return to the trans-Golgi network to repeat the process. However, the mechanism that facilitates ligand binding and dissociation upon changes in pH is not known. We report the crystal structure of the extracytoplasmic domain of the homodimeric cation-dependent MPR in a ligand-free form at pH 6.5. A comparison of the ligand-bound and ligand-free structures reveals a significant change in quaternary structure as well as a reorganization of the binding pocket, with the most prominent change being the relocation of a loop (residues Glu(134)-Cys(141)). The movements involved in the bound-to-free transition of the cation-dependent MPR are reminiscent of those of the oxy-to-deoxy hemoglobin transition. These results allow us to propose a mechanism by which the receptor regulates its ligand binding upon changes in pH; the pK(a) of Glu(133) appears to be responsible for ligand release in the acidic environment of the late endosomal compartment, and the pK(a) values of the sugar phosphate and His(105) are accountable for its inability to bind ligand at the cell surface where the pH is about 7.4.
authors.affiliation: Department of Biochemistry, Medical College of Wisconsin, Milwaukee, Wisconsin 53226, USA.
authors.forename: Linda J
authors.initials: LJ
authors.lastname: Olson
authors.forename: Jian
authors.initials: J
authors.lastname: Zhang
authors.forename: Nancy M
authors.initials: NM
authors.lastname: Dahms
authors.forename: Jung-Ja P
authors.initials: JJ
authors.lastname: Kim
mesh_terms: D000595:Amino Acid Sequence; D001665:Binding Sites; D002942:Circular Dichroism; D003545:Cysteine / Q000737:chemistry; D003593:Cytoplasm / Q000378:metabolism; D019281:Dimerization; D011992:Endosomes / Q000378:metabolism; D018698:Glutamic Acid / Q000737:chemistry; D006639:Histidine / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D008024:Ligands; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010710:Phosphates / Q000737:chemistry; D011485:Protein Binding; D011487:Protein Conformation; D017510:Protein Folding; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D017527:Receptor, IGF Type 2 / Q000737:chemistry* / Q000378:metabolism; D017386:Sequence Homology, Amino Acid
chemical_list: D008024:Ligands; D010710:Phosphates; D017527:Receptor, IGF Type 2; D018698:Glutamic Acid; D006639:Histidine; D003545:Cysteine