NMR solution structure of the receptor binding domain of human alpha(2)-macroglobulin

Human alpha(2)-macroglobulin-proteinase complexes bind to their receptor, t he low density lipoprotein receptor-related protein (LRP), through a discre te 138-residue C-terminal receptor binding domain (RBD), which also binds t o the beta-amyloid peptide. We have used NMR spectroscopy on recombinantly expressed uniformly C-13/N-15-labeled human RED to determine its three-dime nsional structure in solution, Human RED is a sandwich of two antiparallel beta-sheets, one four-strand and one five-strand, and also contains one alp ha-helix of 2.5 turns and an additional 1-turn. helical region. The princip al alpha-helix contains two lysine residues on the outer face that are know n to be essential for receptor binding. A calcium binding site (K-d similar to 11 mM) is present in the loop region at one end of the beta-sandwich, C alcium binding principally affects this loop region and does not significan tly perturb the stable core structure of the domain. The structure and NMR. assignments will enable us to examine in solution specific binding of RED to domains of the receptor and to beta-amyloid peptide.