Structural Insights into Aggregation Mechanism of Immunoglobulin Light Chain Variable Domain

Abstract

Light chain (AL) or systemic amyloidosis is an amyloid disease resulting from an over production of immunoglobulin light chain (IgG) due to abnormal proliferation of monoclonal plasma B cells. These abundant circulating light chains misfold and gets deposited in the form of insoluble fibrils in the extracellular space of various organs leading to their impairment. AL amyloidosis is further complicated by the fact that each patient involving a similar or different organ often tend to present a slightly different protein due to somatic hyper mutation that might aggregate following different kinetics. Little is known about the mechanism of amyloid assembly in AL amyloidosis. Investigation of the aggregation pathways and stages is especially important as many of these involve intermediates structures that could be cytotoxic and play a critical role in the initiation of amyloid assembly. In this work, we discuss the mechanism of in vitro fibril formation in germline (wild type/control) protein κI O18/O8 and its amyloidogenic mutants AL-09 and AL-12. Both amyloidogenic proteins aggregate significantly faster as compared to germline protein with similar rates of aggregation for both AL-09 and AL-12. We also observed that during early stages of aggregation, AL-09 rapidly forms transient low molecular weight oligomers that polymerize into mature fibrils. The single restorative mutation AL-09 H87Y (restoring the amino acid in the germline sequence) alters the aggregation kinetics of AL-09. As compared to AL-09, the rate of aggregation in AL-09 H87Y drastically decreases and the low molecular weight oligomers disappear completely. On the contrary, AL-12 appears to aggregate by polymerization of soluble protein without involving any oligomeric intermediates. The single restorative mutation of AL-12 (AL-12 R65S) to its germline sequence had no effect and appears to have very similar rates of aggregation compared to AL-12.