A conformational fingerprint for amyloidogenic light chains

Immunoglobulin light chain amyloidosis (AL) shares with multiple myeloma (MM) the overproduction of one clonal light chain (LC), but whereas in MM patients LC molecules remain soluble in circulation, AL LCs misfold into toxic soluble species and amyloid fibrils that accumulate in internal organs, leading to completely different clinical manifestations. The large sequence variability of LCs has hampered our understanding of the mechanism leading to LC aggregation. Nevertheless, some biochemical properties associated with AL-LC are emerging. The stability of the dimeric LCs seems to play a role, but conformational dynamics and susceptibility to proteolysis have been identified as biophysical parameters that, under native conditions, can better distinguish AL-LCs from LCs found in MM. In this study, our goal was to delineate a conformational fingerprint that could discriminate AL from MM LCs. By subjecting four AL and two MM LCs to in vitro analysis under native conditions using small-angle X-ray scattering (SAXS), we observed that the AL LCs exhibited a slightly larger radius of gyration and greater deviation from the experimentally determined structure, indicating enhanced conformational dynamics. Integrating SAXS with molecular dynamics (MD) simulations to generate a conformational ensemble revealed that LCs can adopt multiple states, with VL and CL domains either bent or straight. AL-LCs favored a distinct state in which both domains were in a straight conformation, maximizing solvent accessibility at their relative interfaces. This unique conformation was experimentally validated by hydrogen-deuterium exchange mass spectrometry (HDX-MS). Such findings reconcile a wealth of experimental observations and provide a precise structural target for drug design investigations.