A Single Mutation at the Sheet Switch Region Results in Conformational Changes Favoring 6 Light-Chain Fibrillogenesis

• Published in: Journal of molecular biology Vol. 396; no. 2
• Main Authors: Hernández-Santoyo, A., Del Pozo Yauner, L, Fuentes-Silva, D, Ortiz, E, Rudiño-Piñera, E, Sánchez-López, R, Horjales, E, Becerril, B, Rodríguez-Romero, A
• Format: Journal Article
• Language: English
• Published: United States 01.01.2010
• Subjects: BASIC BIOLOGICAL SCIENCES; CAPACITY; CHAINS; CONFORMATIONAL CHANGES; CRYSTAL STRUCTURE; DEPOSITION; DIMERS; DISEASES; GENERAL AND MISCELLANEOUS//MATHEMATICS, COMPUTING, AND INFORMATION SCIENCE; IN VITRO; MONOMERS; MUTANTS; MUTATIONS; national synchrotron light source; PROTEINS; RESIDUES; STABILITY; THERMODYNAMICS
• ISSN: 0022-2836; 1089-8638
• DOI: 10.1016/j.jmb.2009.11.038

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 {lambda} chains, particularly those belonging to the {lambda}6 type, are overrepresented. Here, we report the thermodynamic and in vitro fibrillogenic properties of several mutants of the {lambda}6 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, P2{sub 1}2{sub 1}2{sub 1} and C222{sub 1}. 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 C222{sub 1} lattice showed the establishment of intermolecular {beta}-{beta} interactions that involved the N-terminus and {beta}-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 {lambda}6 LCs.