Formation of critical oligomers is a key event during conformational transition of recombinant syrian hamster prion protein

We have investigated the conformational transition and aggregation process of recombinant Syrian hamster prion protein (SHaPrP90-232) by Fourier transform infrared spectroscopy, circular dichroism spectroscopy, light scattering, and electron microscopy under equilibrium and kinetic conditions. SHaPrP90-232 showed an infrared absorbance spectrum typical of proteins with a predominant alpha-helical structure both at pH 7.0 and at pH 4.2 in the absence of guanidine hydrochloride. At pH 4.2 and destabilizing conditions (0.3-2 m guanidine hydrochloride), the secondary structure of SHaPrP90-232 was transformed to a strongly hydrogen-bonded, most probably intermolecularly arranged antiparallel beta-sheet structure as indicated by dominant amide I band components at 1620 and 1691 cm-1. Kinetic analysis of the transition process showed that the decrease in alpha-helical structures and the increase in beta-sheet structures occurred concomitantly according to a bimolecular reaction. However, the concentration dependence of the corresponding rate constant pointed to an apparent third order reaction. No beta-sheet structure was formed within the dead time (190 ms) of the infrared experiments. Light scattering measurements revealed that the structural transition of SHaPrP90-232 was accompanied by formation of oligomers, whose size was linearly dependent on protein concentration. Extrapolation to zero protein concentration yielded octamers as the smallest oligomers, which are considered as "critical oligomers." The small oligomers showed spherical and annular shapes in electron micrographs. Critical oligomers seem to play a key role during the transition and aggregation process of SHaPrP90-232. A new model for the structural transition and aggregation process of the prion protein is described.