Mechanistic insights into the aggregation pathway of an immunoglobulin light chain protein

Systemic antibody light chains (AL) amyloidosis is characterized by deposition of amyloid fibrils derived from a particular antibody light chain protein. In addition, soluble oligomeric AL protein has a direct cytotoxic effect on cardiomyocytes prior to organ malfunction and amyloid formation. The structure of these oligomers and the mechanism of toxicity is, however, not understood. In this work, we focus on the patient derived λ-III light chain variable domain FOR005. This protein is mutated at five positions with respect to the closest germline protein GL. In addition, we investigate the single point mutant FOR005-R49G and GL. Using solution-state NMR spectroscopy, we were able to follow the individual steps involved in protein misfolding from the native state to the amyloid fibril structure at atomic resolution for the first time. We show that unfavorable mutations in the complementary determining regions introduce a strain in the protein structure which induces partial unfolding of the native state. Subsequently, the protein converts into a high molecular weight, oligomeric, molten globule like structure. We demonstrate that this transition is driven by electrostatic interactions. The high local concentration of aggregation prone regions in the oligomer finally catalyzes the conversion into amyloid fibrils. We find that the topology of the aggregated state is determined by balanced electrostatic interactions in the core of the fibril, resulting in an anti-parallel arrangement of the beta-sheets around the conserved disulfide bond.