AMYLOID FIBRIL FORMATION REQUIRES A CHEMICALLY DISCRIMINATING NUCLEATION EVENT - STUDIES OF AN AMYLOIDOGENIC SEQUENCE FROM THE BACTERIAL PROTEIN OSMB

The sequence of the Escherichia coli OsmB protein was found to resemble that of the C-terminal region of the beta amyloid protein of Alzheimers's disease, which seems to be the major determinant of its unusual structural and solubility properties. A peptide corresponding to residues 28-44 of the OsmB protein was synthesized, and its conformational properties and aggregation behavior were analyzed. The peptide OsmB(28-44) was shown to form amyloid fibrils, as did two sequence analogs designed to test the sequence specificity of fibril formation. These fibrils bound Congo red, and two of the peptides showed birefringence. The peptide fibrils were analyzed by electron microscopy and Fourier transform infrared spectroscopy. Subtle differences were observed which were not interpretable at the molecular level. The rate of fibril formation by each peptide was followed by monitoring the turbidity of supersaturated aqueous solutions. The kinetics of aggregation were characterized by a delay period during which the solution remained clear, followed by a nucleation event which led to a growth phase, during which the solution became viscous and turbid due to the presence of insoluble fibrils. The observation of a kinetic barrier to aggregation is typical of a crystallization event. The delay period could be eliminated by seeding the supersaturated solution with previously formed fibrils. Each peptide could be nucleated by fibrils formed from that same peptide, but not by fibrils from closely related sequences, suggesting that fibril growth requires specific hydrophobic interactions. It appears likely that this repeated sequence motif which comprises most of the OsmB protein sequence, dictates the structure and possibly the function of that protein. While amyloid formation was shown not to require this sequence, similarities in sequence and composition between OsmB and the beta amyloid protein may be significant. Furthermore, these studies demonstrate that amyloid fibril formation is chemically discriminating, and that nucleation of fibril formation is the rate-limiting step in amyloidogenesis. This finding has important ramifications for the understanding of amyloid formation in Alzheimer's disease.