Characterization of the oligomeric state of amyloid proteins
Conversion of soluble amyloid proteins into their fibrillar form has been postulated to progress through various intermediate stages which are only stable transiently, and the final fibrillar form is the most stable and thus the easiest to characterize. Oligomers have been characterized as two types namely soluble oligomers and the fibrillar oligomers. The soluble oligomers are intermediates that form as the soluble monomers progress into mature fibrils. Previous attempts at characterizing these oligomers have been limited to indirect methods such as circular dichroism, FTIR and electron microscopy. But the recent characterization of an 11-residue peptide from alpha-B-crystallin by X-ray crystallography is the first detailed structure of a toxic amyloidogenic peptide segment in its oligomeric state. We hypothesize that this structure, termed cylindrin, is a common oligomeric state of other amyloid proteins. Using this structure as a model, we identified various peptide segments in more than five other amyloid forming proteins including alpha synuclein, tau, abeta and IAPP. In this work I characterized five segments from alpha synuclein and superoxide dismutase 1 (SOD) that were predicted to form cylindrins. The segments were found to have beta sheet structure by circular dichroism and a dimeric oligomeric state by static light scattering. They were also found to be toxic to cultured cells. This study provides preliminary evidence of cylindrin like oligomeric state of amyloid segments and further studies are required to validate our hypothesis.