NMR solution structure of complement-like repeat CR3 from the low density lipoprotein receptor-related protein - Evidence for specific binding to thereceptor binding domain of human alpha(2)-macroglobulin

We have used NMR methods to determine the structure of the calcium complex of complement-like repeat 3 (CR3) from the low density lipoprotein receptor -related protein (LRP) and to examine its specific interaction with the rec eptor binding domain of human alpha(2)-macroglobulin. CR3 is one of eight r elated repeats that constitute a major ligand binding region of LRP. The st ructure is very similar in overall fold to homologous complement-like repea t CR8 from LRP and complement-like repeats LB1, LB2, and LB5 from the low d ensity lipoprotein receptor and contains a short two-strand antiparallel be ta-sheet, a one turn alpha-helix, and a high affinity calcium site with coo rdination from four carboxyls and two backbone carbonyls, The surface elect rostatics and topography are, however, quite distinct from each of these ot her repeats. Two-dimensional H-1,N-15-heteronuclear single quantum coherenc e spectra provide evidence for a specific, though relatively weak (K-d simi lar to 140 mu M), interaction between CR3 and human alpha 2-macroglobulin r eceptor binding domain that involves a contiguous patch of surface residues in the central region of CR3. This specific interaction is consistent with a mode of LRP binding to ligands that uses contributions from more than on e domain to generate a wide array of different binding sites, each with ove rall high affinity.