title: Ca2+ binding to complement-type repeat domains 5 and 6 from the low-density lipoprotein receptor-related protein.
abstract: BACKGROUND
  The binding of ligands to clusters of complement-type repeat (CR)-domains in proteins of the low-density lipoprotein receptor (LDLR) family is dependent on Ca2+ ions. One reason for this cation requirement was identified from the crystal structure data for a CR-domain from the prototypic LDLR, which showed the burial of a Ca2+ ion as a necessity for correct folding and stabilization of this protein module. Additional Ca2+ binding data to other CR-domains from both LDLR and the LDLR-related protein (LRP) have suggested the presence of a conserved Ca2+ cage within CR-domains from this family of receptors that function in endocytosis and signalling.
  
  
  RESULTS
  We have previously described the binding of several ligands to a fragment comprising the fifth and the sixth CR-domain (CR56) from LRP, as well as qualitatively described the binding of Ca2+ ions to this CR-domain pair. In the present study we have applied the rate dialysis method to measure the affinity for Ca2+, and show that CR56 binds 2 Ca2+ ions with an average affinity of KD = 10.6 microM, and there is no indication of additional Ca2+ binding sites within this receptor fragment.
  
  
  CONCLUSIONS
  Both CR-domains of CR56 bind a single Ca2+ ion with an affinity of 10.6 microM within the range of affinities demonstrated for several other CR-domains.
authors.affiliation: Laboratory of Gene Expression, Department of Molecular and Structural Biology, University of Aarhus, DK-8000 Aarhus C, Denmark. o.andersen@mdc-berlin.de
authors.forename: Olav M
authors.initials: OM
authors.lastname: Andersen
authors.forename: Henrik
authors.initials: H
authors.lastname: Vorum
authors.forename: Bent
authors.initials: B
authors.lastname: Honoré
authors.forename: Hans C
authors.initials: HC
authors.lastname: Thøgersen
mesh_terms: D000595:Amino Acid Sequence; D001665:Binding Sites; D002118:Calcium / Q000737:chemistry / Q000378:metabolism*; D002135:Calcium-Binding Proteins / Q000737:chemistry* / Q000378:metabolism; D003165:Complement System Proteins / Q000737:chemistry*; D026502:LDL-Receptor Related Proteins / Q000737:chemistry* / Q000378:metabolism; D008969:Molecular Sequence Data; D017434:Protein Structure, Tertiary
chemical_list: D002135:Calcium-Binding Proteins; D026502:LDL-Receptor Related Proteins; D003165:Complement System Proteins; D002118:Calcium