As a class of molecular chaperones, small heat shock proteins (sHsps) usually exist as multi-subunit spherical oligomers. In this study, we report that AgsA, a sHsp of Salmonella enterica serovar Typhimurium, spontaneously forms fibrils in vitro. These fibrils tend to be formed at elevated temperature and do not share the characteristics of amyloid. Interestingly, the fibril-forming AgsA is able to suppress the dithiothreitol-induced aggregation of insulin efficiently within a certain range of temperature. During this process, AgsA fibrils disappear and spherical complexes form between AgsA and insulin molecules. These data suggest that AgsA fibrils may represent a distinctive type of structural and functional form of sHsp from spherical oligomers. Our study provides new insights into sHsp structures and chaperone functions.
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