Formation of amyloid fibrils by peptides derived from the bacterial cold shock protein CspB

Three peptides covering the sequence regions corresponding to the first two (CspB-1), the first three (CspB-2), and the last two (CspB-3) beta-strands of CspB, the major cold shock protein of Bacillus subtilis, have been synt hesized and analyzed for their conformations in solution and for their prec ipitation behavior. The peptides are nearly insoluble in water, but highly soluble in aqueous solutions containing 50% acetonitrile (pH 4.0). Upon shi fts of the solvent condition toward lower or higher acetonitrile concentrat ions, the peptides all form fibrils resembling those observed in amyloid as sociated diseases. These fibrils have been identified and characterized by electron microscopy, binding of the dye congo red, and X-ray fiber diffract ion. Characterization of the peptides in solution by circular dichroism and NMR spectroscopy shows that the formation of these fibrils does not requir e specific preformed secondary structure in the solution state species. Whi le the majority of the soluble fraction of each peptide is monomeric and un structured, different types of structures including alpha-helical, beta-she et, and random coil conformations are observed under conditions that eventu ally lead to fibril formation. We conclude that the absence of tertiary con tacts under solution conditions where binding interactions between peptide units are still favorable is a crucial requirement for amyloid formation. T hus, fragmentation of a sequence, like partial chemical denaturation or mut ation, can enhance the capacity of specific protein sequences to form such fibrils.