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, ran ging 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 a nalyzed the stability and oligomeric structure of murine Hsp25 and its corr elation with function. Upon unfolding, the tertiary and quaternary structur e of Hsp25 is rapidly lost, whereas the secondary structure remains remarka bly stable. Unfolding is completely reversible, leading to native hexadecam eric structures. These oligomers are in a concentration-dependent equilibri um with tetramers and dimers, indicating that tetramers assembled from dime rs represent the basic building blocks of Hsp25 oligomers. At high temperat ures, the Hsp25 complexes increase in molecular mass, consistent with the a ppearance of "heat shock granules" in vivo after heat treatment. This high molecular mass "heat shock form" of Hsp25 is in a slow equilibrium with hex adecameric Hsp25. Thus, it does not represent an off-pathway reaction. Inte restingly, the heat shock form exhibits unchanged chaperone activity even a fter incubation at 80 degrees C. We conclude that Hsp25 is a dynamic tetram er of tetramers with a unique ability to refold and reassemble into its act ive quaternary structure after denaturation. So-called heat shock granules, which have been reported to appear in response to stress, seem to represen t a novel functional species of Hsp25.