NMR solution structure of the receptor binding domain of human α2-macroglobulin

Human alpha(2)-macroglobulin-proteinase complexes bind to their receptor, the low density lipoprotein receptor-related protein (LRP), through a discrete 138-residue C-terminal receptor binding domain (RBD), which also binds to the beta-amyloid peptide. We have used NMR spectroscopy on recombinantly expressed uniformly C-13/N-15-labeled human RED to determine its three-dimensional structure in solution, Human RED is a sandwich of two antiparallel beta-sheets, one four-strand and one five-strand, and also contains one alpha-helix of 2.5 turns and an additional 1-turn. helical region. The principal alpha-helix contains two lysine residues on the outer face that are known 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, Calcium binding principally affects this loop region and does not significantly 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.