Stepwise dynamics of epitaxially growing single amyloid fibrils

The assembly mechanisms of amyloid fibrils, tissue deposits in a variety of degenerative diseases, is poorly understood. With a simply modified application of the atomic force microscope, we monitored the growth, on mica surface, of individual fibrils of the amyloid β25-35 peptide with near-subunit spatial and subsecond temporal resolution. Fibril assembly was polarized and discontinuous. Bursts of rapid (up to 300-nm⁻¹) growth phases that extended the fibril by [almost equal to]7 nm or its integer multiples were interrupted with pauses. Stepwise dynamics were also observed for amyloid β1-42 fibrils growing on graphite, suggesting that the discontinuous assembly mechanisms may be a general feature of epitaxial amyloid growth. Amyloid assembly may thus involve fluctuation between a fast-growing and a blocked state in which the fibril is kinetically trapped because of intrinsic structural features. The used scanning-force kymography method may be adapted to analyze the assembly dynamics of a wide range of linear biopolymers.