A Conservative Point Mutation in a Dynamic Antigen-binding Loop of Human Immunoglobulin λ6 Light Chain Promotes Pathologic Amyloid Formation

Immunoglobulin light chain (LC) amyloidosis (AL) is a life-threatening human disease wherein free mono-clonal LCs deposit in vital organs. To determine what makes some LCs amyloidogenic, we explored patient-based amyloidogenic and non-amyloidogenic recombinant LCs from the lambda 6 subtype prevalent in AL. Hydrogen-deuterium exchange mass spectrometry, structural stability, proteolysis, and amyloid growth studies revealed that the antigen-binding CDR1 loop is the least protected part in the variable domain of lambda 6 LC, particularly in the AL variant. N32T substitution in CRD1 is identified as a driver of amyloid formation. Substitution N32T increased the amyloidogenic propensity of CDR1 loop, decreased its protection in the native structure, and accelerated amyloid growth in the context of other AL substitutions. The destabilizing effects of N32T propagated across the molecule increasing its dynamics in regions similar to 30 angstrom away from the substitution site. Such striking long-range effects of a conservative point substitution in a dynamic surface loop may be relevant to Ig function. Comparison of patient-derived and engineered proteins showed that N32T interactions with other substitution sites must contribute to amyloidosis. The results suggest that CDR1 is critical in amyloid formation by other lambda 6 LCs. (C) 2021 Elsevier Ltd. All rights reserved.

Automated summary

This study identifies the N32T substitution in the CRD1 of lambda 6 LC as a driver of amyloid formation, which increases the amyloidogenic propensity of the CDR1 loop and accelerates amyloid growth. The destabilizing effects of N32T propagate across the molecule and impact regions up to 30 angstrom away from the substitution site, suggesting long-range effects of a conservative point substitution in a dynamic surface loop relevant to Ig function. Comparisons between patient-derived and engineered proteins indicate that interactions between N32T and other substitution sites contribute to amyloidosis, highlighting the critical role of CDR1 in amyloid formation by lambda 6 LCs.