Mechanistic Insights Into Metal Binding in the Aggregation Pathway of an Immunoglobulin Light Chain Protein

Cardiac antibody light chain (AL) amyloidosis is characterized by the deposition of light chain (LC) variable domain (V_L) protein in the heart. Transition metals such as copper and zinc have been suggested to enhance the toxicity of circulating soluble LCs. We show here how divalent metal ions influence the aggregation behavior of the patient-derived λ-III AL variable domain FOR005. We characterize metal binding across dimeric, monomeric, oligomeric, and fibril aggregation states, using solution- and MAS solid-state NMR spectroscopy, as well as molecular dynamics (MD) simulations. Zn²⁺ and Cu²⁺ reduce the lag time for fibril formation. In the dimer, the Cu²⁺ binding site involves the side chains of residues D91 from protomer 1 and H96 from protomer 2. The accelerated aggregation kinetics induced by Cu²⁺ arises from differential stabilization of oligomeric intermediate states, either through depletion of off-pathway intermediates or shortened lifetimes of on-pathway species. We find that Cu²⁺ does not bind to a specific site but interacts nonspecifically with the protein fibrils. We hypothesize that a decrease of the cytosolic free pool of ions perturbs the cellular ion homeostasis in cardiomyocytes which in turn can result in dysregulated metabolic function.