Ultrastructural organization of amyloid fibrils by atomic force microscopy

Atomic force microscopy has been employed to investigate the structural org anization of amyloid fibrils produced in vitro from three very different po lypeptide sequences. The systems investigated are a 10-residue peptide deri ved from the sequence of transthyretin, the 90-residue SH3 domain of bovine phosphatidylinositol-3'-kinase, and human wild-type lysozyme, a 130-residu e protein containing four disulfide bridges. The results demonstrate distin ct similarities between the structures formed by the different classes of f ibrils despite the contrasting nature of the polypeptide species involved. SH3 and lysozyme fibrils consist typically of four protofilaments, exhibiti ng a left-handed twist along the fibril axis. The substructure of TTR10-19 fibrils is not resolved by atomic force microscopy and their uniform appear ance is suggestive of a regular self-association of very thin filaments. We propose that the exact number and orientation of protofilaments within amy loid fibrils is dictated by packing of the regions of the polypeptide chain s that are not directly involved in formation of the cross-beta core of the fibrils. The results obtained for these proteins, none of which is directl y associated with any human disease, are closely similar to those of diseas e-related amyloid fibrils, supporting the concept that amyloid is a generic structure of polypeptide chains. The detailed architecture of an individua l fibril, however, depends on the manner in which the protofilaments assemb le into the fibrillar structure, which in turn is dependent on the sequence of the polypeptide and the conditions under which the fibril is formed.