Role of Zn(II) in the aggregation of the 6aJL2R24G protein: Experimental and theoretical approach.

Light chain amyloidosis is a conformational disease, and one of the most common systemic amyloidosis. It is characterized by the deposition of amyloid aggregates of immunoglobulin light chains in organs and tissues. 6aJL2R24G is a recombinant variant of the λ6a germline protein, a germline present in 25 % of the amyloid-associated λ6a light chain amyloidosis cases. In this study, using spectroscopic and computational methodologies, we found that the interaction of this protein with Zn(II) accelerates amyloid fibril initiation and slows down elongation, without altering the fiber morphology. Also, neither the thermal stability nor the secondary structure are altered by the interaction with the metal ion, as measured by circular dichroism. Isothermal calorimetry titration showed that the protein has two binding sites with affinities in the micromolar range. Molecular dynamics simulations suggest that the interaction between 6aJL2R24G and Zn(II) destabilizes strand A, strand G and the EF loop, making the protein more prone to initiate the formation of amyloid fibers. Furthermore, models of 6aJL2R24G dimers and tetramers with Zn(II) suggest that Zn(II) promotes the association of the proteins, involving histidine, aspartate, and glutamate residues, with multiple different geometries, effectively raising the local protein concentration and promoting seed formation, but adopting conformations different from the one required for further monomer addition to the fiber.