Immunoglobulin Light Chains Form an Extensive and Highly Ordered Fibril Involving the N- and C-Termini

Light-chain (AL)-associated amyloidosis is a systemic disorder involving the formation and deposition of immunoglobulin AL fibrils in various bodily organs. One severe instance of AL disease is exhibited by the patient-derived variable domain (V_L) of the light chain AL-09, a 108 amino acid residue protein containing seven mutations relative to the corresponding germline protein, κI O18/O8 V_L. Previous work has demonstrated that the thermodynamic stability of native AL-09 V_L is greatly lowered by two of these mutations, Y87H and N34I, whereas a third mutation, K42Q, further increases the kinetics of fibril formation. However, detailed knowledge regarding the residues that are responsible for stabilizing the misfolded fibril structure is lacking. In this study, using solid-state NMR spectroscopy, we show that the majority of the AL-09 V_L sequence is immobilized in the fibrils and that the N- and C-terminal portions of the sequence are particularly well-structured. Thus, AL-09 V_L forms an extensively ordered and β-strand-rich fibril structure. Furthermore, we demonstrate that the predominant β-sheet secondary structure and rigidity observed for in vitro prepared AL-09 V_L fibrils are qualitatively similar to those observed for AL fibrils extracted from postmortem human spleen tissue, suggesting that this conformation may be representative of a common feature of AL fibrils.