The Critical Role of the Constant Region in Thermal Stability and Aggregation of Amyloidogenic Immunoglobulin Light Chain

Light chain (LC) amyloidosis (AL) is a fatal disease in which immunoglobulin LC deposit as fibrils. Although the LC amyloid-forming propensity is attributed primarily to the variable region, fibrils also contain full-length LC comprised of variable-joining (V-L) and constant (C-L) regions. To assess the role of C-L in fibrillogenesis, we compared the thermal stability of full-length LC and corresponding V-L and C-L fragments. Protein unfolding and aggregation were monitored by circular dichroism and light scattering. A full-length lambda 6 LC purified from urine of a patient with AL amyloidosis showed irreversible unfolding coupled to aggregation. The transition temperature decreased at slower heating rates, indicating kinetic effects. Next, we studied five recombinant lambda 6 proteins: full-length amyloidogenic LC, its V-L, germline LC, germline V-L, and C-L. Amyloidogenic and germline proteins showed similar rank order of stability, V-L < LC < C-L; hence, in the full-length LC, V-L destabilizes C-L. Amyloidogenic proteins were less stable than their germline counterparts, suggesting that reduction in V-L stability destabilizes the full-length LC. Thermal unfolding of the full-length amyloidogenic and germline LC required high activation energy and involved irreversible aggregation, yet the unfolding of the isolated V-L and C-L fragments was partially reversible. Therefore, compared to their fragments, full-length LCs are more likely to initiate aggregation during unfolding and provide a template for the V-L deposition. The kinetic barrier for this aggregation is regulated by the stability of the V-L region. This represents a paradigm shift in AL fibrillogenesis and suggests C-L region as a potential therapeutic target.