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In this study, we expressed, purified and characterized small heat shock proteins (sHsp) from a magnetotactic bacterium, Magnetospirillum magneticum AMB-1, Hsp17.5, Hsp17.8 and Hsp18.3. They all showed molecular chaperone activity to protect citrate synthase from thermal aggregation at 45°C. The oligomeric states were examined by size exclusion chromatography and electron microscopy. Hsp17.8 exists as a spherical oligomer like other sHsps, and the oligomer reversibly dissociates to small conformers, probably dimers, at elevated temperatures. On the contrary, Hsp 17.5 and Hsp 18.3 take large filamentous conformations. A similar structure was observed in an sHsp of Sulfolobus tokodaii, StHsp19.7, but it did not exhibit molecular chaperone activity. Hsp17.5 and Hsp18.3 changed their oligomeric states according to the elevation of temperature. Among them, Hsp17.5 exhibited reversible dissociation. This is the first report that the filamentous sHsp exhibit molecular chaperone activity with the change of oligomeric states. [ABSTRACT FROM AUTHOR]