Atomic-resolution map of the interactions between an amyloid inhibitor protein and amyloid β(Aβ) peptides in the monomer and protofibril states

Self-association of amyloid beta (A beta) peptides is a hallmark of Alzheimer's disease and serves as a general prototype for amyloid formation. A key endogenous inhibitor of A beta self-association is human serum albumin (HSA), which binds similar to 90% of plasma A beta. However, the exact molecular mechanism by which HSA binds A beta monomers and protofibrils is not fully understood. Here, using dark-state exchange saturation transfer NMR and relaxation experiments complemented by morphological characterization, we mapped the HSA-A beta interactions at atomic resolution by examining the effects ofHSAonA beta monomers and soluble high-molecular weight oligomeric protofibrils. We found that HSA binds both monomeric and protofibrillar A beta, but the affinity of HSA for A beta monomers is lower than for A beta protofibrils (K-d values are submillimolar rather than micromolar) yet physiologically relevant because of the similar to 0.6-0.7 mM plasma HSA concentration. In both A beta-protofibrils and monomers, HSA targets key A beta self-recognition sites spanning the beta strands found in cross-beta protofibril structures, leading to a net switch from direct to tethered contacts between the monomeric A beta and the protofibril surface. These HSAA beta interactions are isoform-specific, because the HSA affinity of A beta monomers is lower for A beta (1-42) than for A beta (1-40). In addition, the HSA-induced perturbations of the monomer/protofibrils pseudo-equilibrium extend to the C-terminal residues in the A beta (1-42) isoform but not in A beta (1-40). These results provide an unprecedented view of how albumin interacts with A beta and illustrate the potential of dark-state exchange saturation transfer NMR in mapping the interactions between amyloid-inhibitory proteins and amyloidogenic peptides.