Pressure-dissociable reversible assembly of intrinsically denatured lysozyme is a precursor for amyloid fibrils.

Abstract

Although a diversity of proteins is known to form amyloid fibers, their common mechanisms are not clear. Here, we show that an intrinsically unfolded protein (U), represented by a disulfide-deficient variant of hen lysozyme with no tertiary structure, forms an amyloid-like fibril after prolonged incubation. Using variable pressure NMR along with sedimentation velocity, circular dichroism, and fluorescence measurements, we show that, before the fibril formation, the protein forms a pressure-dissociable, soluble assemblage (U'(n)) with a sedimentation coefficient of 17 S and a rich intermolecular beta-sheet structure. The reversible assemblage is characterized with a Gibbs energy for association of -23.3 +/- 0.8 kJ.mol(-1) and a volume increase of 52.7 +/- 11.3 ml.mol(-1) per monomer unit, and involves preferential interaction of hydrophobic residues in the initial association step. These results indicate that amyloid fibril formation can proceed from an intrinsically denatured protein and suggest a scheme N <==>U <==>U'(n)-->fibril as a common mechanism of fibril formation in amyloidogenic proteins, where two-way arrows represent reversible processes, one-way arrow represents an irreversible process, and N, U, and U'(n)represent, respectively, the native conformer, the unfolded monomeric conformer, and the soluble assemblage of unfolded conformers.