Aggregation of S6 in a quasi-native state by sub-micellar SDS

Anionic surfaces promote protein fibrillation in vitro and in vivo. Monomeric SDS has also been shown to stimulate this process. We describe the dynamics of conformational changes and aggregative properties of the model protein S6 at sub-micellar SDS concentrations. S6 exhibits a rich and pH-sensitive diversity in conformational changes around 0.2-2 mM SDS, in which several transitions occur over time scales spanning milliseconds to hours. Monomeric SDS readily precipitates S6 within minutes at pH-values of 5 and below to form states able to bind the fibril-specific dye thioflavin T. At pH 5.5, the process is much slower and shows a mutagenesis-sensitive lag, leading to different forms of organized but not classically fibrillar aggregates with native-like levels of secondary structure, although the tertiary structure is significantly rearranged. The slow aggregation process may be linked to conformational changes that occur at the second-time scale in the same SDS concentration range, leading to an altered structure, possibly with unfolding around the C-terminal helix. The S6 aggregates may be differently trapped states, equivalent to pre-fibrillar structures seen at early stages in the fibrillation process for other proteins. The low quantities of anionic species required suggest that the aggregates may have parallels in vivo.