A Single Mutation at the Sheet Switch Region Results in Conformational Changes Favoring λ6 Light-Chain Fibrillogenesis

Systemic amyloid light-chain (LC) amyloidosis is a disease process characterized by the pathological deposition of monoclonal LCs in tissue. All LC subtypes are capable of fibril formation although λ chains, particularly those belonging to the λ6 type, are overrepresented. Here, we report the thermodynamic and in vitro fibrillogenic properties of several mutants of the λ6 protein 6aJL2 in which Pro7 and/or His8 was substituted by Ser or Pro. The H8P and H8S mutants were almost as stable as the wild-type protein and were poorly fibrillogenic. In contrast, the P7S mutation decreased the thermodynamic stability of 6aJL2 and greatly enhanced its capacity to form amyloid-like fibrils in vitro. The crystal structure of the P7S mutant showed that the substitution induced both local and long-distance effects, such as the rearrangement of the VL (variable region of the light chain)–VL interface. This mutant crystallized in two orthorhombic polymorphs, P212121 and C2221. In the latter, a monomer that was not arranged in the typical Bence–Jones dimer was observed for the first time. Crystal-packing analysis of the C2221 lattice showed the establishment of intermolecular β–β interactions that involved the N-terminus and β-strand B and that these could be relevant in the mechanism of LC fibril formation. Our results strongly suggest that Pro7 is a key residue in the conformation of the N-terminal sheet switch motif and, through long-distance interactions, is also critically involved in the contacts that stabilized the VL interface in λ6 LCs.