The dynamics of Hsp25 quaternary structure. Structure and function of different oligomeric species.

Small heat shock proteins (sHsps), including alpha-crystallin, represent a conserved and ubiquitous family of proteins. They form large oligomers, ranging in size from 140 to more than 800 kDa, which seem to be important for the interaction with non-native proteins as molecular chaperones. Here we analyzed the stability and oligomeric structure of murine Hsp25 and its correlation with function. Upon unfolding, the tertiary and quaternary structure of Hsp25 is rapidly lost, whereas the secondary structure remains remarkably stable. Unfolding is completely reversible, leading to native hexadecameric structures. These oligomers are in a concentration-dependent equilibrium with tetramers and dimers, indicating that tetramers assembled from dimers represent the basic building blocks of Hsp25 oligomers. At high temperatures, the Hsp25 complexes increase in molecular mass, consistent with the appearance of "heat shock granules" in vivo after heat treatment. This high molecular mass "heat shock form" of Hsp25 is in a slow equilibrium with hexadecameric Hsp25. Thus, it does not represent an off-pathway reaction. Interestingly, the heat shock form exhibits unchanged chaperone activity even after incubation at 80 degrees C. We conclude that Hsp25 is a dynamic tetramer of tetramers with a unique ability to refold and reassemble into its active quaternary structure after denaturation. So-called heat shock granules, which have been reported to appear in response to stress, seem to represent a novel functional species of Hsp25.