Conformational Differences in the Light Chain Constant Domain of Immunoglobulin G and Free Light Chain May Influence Proteolysis in AL Amyloidosis.

Immunoglobulin light chain amyloidosis (AL) is a life-threatening disease caused by the deposition of light chain (LC) and its fragments containing variable (VL) and portions of constant (CL) domains. AL patients feature either monoclonal free LCs (FLCs) circulating as covalent and noncovalent homodimers, or monoclonal immunoglobulin (Ig) wherein the LC and heavy chain (HC) form disulfide-linked heterodimers, or both. The role of full-length Ig in AL amyloidosis is unclear as prior studies focused on FLC or VL domain. We used a mammalian cell-based expression system to generate four AL patient-derived full-length IgGs, two non-AL IgG controls, and six corresponding FLC proteins derived from an IGLV6-57 germline precursor. Comparison of proteins' secondary structure, thermal stability, proteolytic susceptibility, and disulfide link reduction suggested the importance of local vs. global conformational stability. Analysis of IgGs vs. corresponding FLCs using hydrogen-deuterium exchange mass spectrometry revealed major differences in the local conformation/dynamics of the CL domain. In all IgGs vs. FLCs, segments containing β-strand and α-helix βAC-αACBC were more dynamic/exposed while segment βDC-βEC was less dynamic/exposed. Notably, these segments overlapped proteolysis-prone regions whose in vivo cleavage generates LC fragments found in AL deposits. Altogether, the results suggest that preferential cleavage in segments βAC-αACBC of FLC or βDC-βEC of LC in IgG helps generate amyloid protein precursors. We propose that protecting these segments using small-molecule stabilizers, which bind to the interfacial cavities CL-CL in FLC and/or CL-CH1 in IgG, is a potential therapeutic strategy to complement current approaches targeting VL-VL or VL-CL stabilization of LC dimer.