title: Structural basis of transcobalamin recognition by human CD320 receptor.
abstract: Cellular uptake of vitamin B12 (cobalamin) requires capture of transcobalamin (TC) from the plasma by CD320, a ubiquitous cell surface receptor of the LDLR family. Here we present the crystal structure of human holo-TC in complex with the extracellular domain of CD320, visualizing the structural basis of the TC-CD320 interaction. The observed interaction chemistry can rationalize the high affinity of CD320 for TC and lack of haptocorrin binding. The in vitro affinity and complex stability of TC-CD320 were quantitated using a solid-phase binding assay and thermostability analysis. Stable complexes with TC were also observed for the disease-causing CD320ΔE88 mutant and for the isolated LDLR-A2 domain. We also determined the structure of the TC-CD320ΔE88 complex, which revealed only minor changes compared with the wild-type complex. Finally, we demonstrate significantly reduced in vitro affinity of TC for CD320 at low pH, recapitulating the proposed ligand release during the endocytic pathway.
authors.affiliation: Department of Biology, Institute of Molecular Biology and Biophysics, ETH Zurich, 8093 Zurich, Switzerland.
authors.forename: Amer
authors.initials: A
authors.lastname: Alam
authors.affiliation: Department of Biology, Institute of Molecular Biology and Biophysics, ETH Zurich, 8093 Zurich, Switzerland.
authors.forename: Jae-Sung
authors.identifier: http://orcid.org/0000-0001-9163-3433
authors.initials: JS
authors.lastname: Woo
authors.affiliation: Department of Biology, Institute of Molecular Biology and Biophysics, ETH Zurich, 8093 Zurich, Switzerland.
authors.forename: Jennifer
authors.initials: J
authors.lastname: Schmitz
authors.affiliation: Department of Biology, Institute of Molecular Biology and Biophysics, ETH Zurich, 8093 Zurich, Switzerland.
authors.forename: Bernadette
authors.initials: B
authors.lastname: Prinz
authors.affiliation: Department of Chemistry and Applied Biosciences, ETH Zurich, 8093 Zurich, Switzerland.
authors.forename: Katharina
authors.initials: K
authors.lastname: Root
authors.affiliation: Department of Chemistry and Applied Biosciences, ETH Zurich, 8093 Zurich, Switzerland.
authors.forename: Fan
authors.initials: F
authors.lastname: Chen
authors.affiliation: Department of Biology, Institute of Molecular Biology and Biophysics, ETH Zurich, 8093 Zurich, Switzerland.
authors.forename: Joël S
authors.initials: JS
authors.lastname: Bloch
authors.affiliation: Department of Chemistry and Applied Biosciences, ETH Zurich, 8093 Zurich, Switzerland.
authors.forename: Renato
authors.initials: R
authors.lastname: Zenobi
authors.affiliation: Department of Biology, Institute of Molecular Biology and Biophysics, ETH Zurich, 8093 Zurich, Switzerland.
authors.forename: Kaspar P
authors.initials: KP
authors.lastname: Locher
mesh_terms: D015703:Antigens, CD / Q000737:chemistry / Q000378:metabolism*; D018360:Crystallography, X-Ray; D006639:Histidine / Q000378:metabolism; D006801:Humans; D006863:Hydrogen-Ion Concentration; D008958:Models, Molecular; D000072417:Protein Domains; D011956:Receptors, Cell Surface; D013329:Structure-Activity Relationship; D014155:Transcobalamins / Q000737:chemistry* / Q000378:metabolism*
chemical_list: D015703:Antigens, CD; C538894:CD320 protein, human; D011956:Receptors, Cell Surface; D014155:Transcobalamins; D006639:Histidine