Amyloid formation via supramolecular peptide assemblies

Amyloid fibrils have been classically defined as linear, nonbranched polymeric proteins with a cross beta-sheet structure and the ability to alter the optical properties of the amyloid-specific dye Congo Red. Mounting evidence suggests that soluble oligomeric peptide assemblies approximately 2-20 nm in diameter are critical intermediates in amyloid formation. Using a pathogenic prion protein peptide comprised of residues 23-144, we demonstrate that, under quiescent but not agitated conditions, much larger globular assemblies up to 1 mum in diameter are made. These globules precede fibril formation and directly interact with growing fibril bundles. Fibrils made via these large spherical peptide assemblies displayed a remarkable diversity of ultrastructural features. Fibrillization of the Abeta1-40 peptide under similar conditions yielded similar results, suggesting a mechanism of general amyloid formation that can proceed through intermediates much larger than those previously described. Our data suggest that simply changing the physical microenvironment can profoundly influence the mechanism of amyloid formation and yield fibrils with novel ultrastructural properties.