Monitoring the assembly of Ig light-chain amyloid fibrils by atomic force microscopy

Aggregation of Ig light chains to form amyloid fibrils is a characteristic feature of light-chain amyloidosis, a light-chain deposition disease. A rec ombinant variable domain of the light chain SMA was used to form amyloid fi brils in vitro. Fibril formation was monitored by atomic force microscopy i maging. Single filaments 2.4 nm in diameter were predominant at early times ; protofibrils 4.0 nm in diameter were predominant at intermediate times; t ype I and type II fibrils 8.0 nm and 6.0 nm in diameter, respectively, were predominant at the endpoints, The increase in number of fibrils correlated with increased binding of the fluorescent dye thioflavin T. The fibrils an d protofibrils showed a braided structure, suggesting that their formation involves the winding of protofibrils and filaments, respectively. These obs ervations support a model in which two filaments combine to form a protofib ril, two protofibrils intertwine to form a type I fibril, and three filamen ts form a type II fibril.