Calcium coordination and pH dependence of the calcium affinity of ligand-binding repeat CR7 from the LRP. Comparison with related domains from the LRP and the LDL receptor

We have determined the X-ray crystal structure to 1.8 Angstrom resolution of the Ca2+ complex of complement-like repeat 7 (CR7) from the low-density lipoprotein receptor-related protein (LRP) and characterized its calcium binding properties at pH 7.4 and 5. CR7 occurs in a region of the LRP that binds to the receptor-associated protein, RAP, and other protein ligands in a Ca2+-dependent manner. The calcium coordination is identical to that found in LB5 and consists of carboxyls from three conserved aspartates and one conserved glutamate, and the backbone carbonyls of a tryptophan and another aspartate. The overall fold of CR7 is similar to those of CR3 and CR8 from the LRP and LB5 from the LDL receptor, though the low degree of sequence homology of residues not involved in calcium coordination or in disulfide formation results in a distinct pattern of surface residues for each domain, including CRT The thermodynamic parameters for Ca2+ binding at both extracellular and endosomal pHs were determined by isothermal titration calorimetry for CR7 and for related complement-like repeats CR3, CR8, and LB5. Although the drop in pH resulted in a reduction in calcium affinity in each case, the changes were very variable in magnitude, being as low as a 2-fold reduction for CR3. This suggests that a pH-dependent change in calcium affinity alone cannot be responsible for the release of bound protein ligands from the LRP at the pH prevailing in the endosome, which in turn requires one or more other pH-dependent effects for regulating protein ligand release.