Chaperone activity of cytosolic small heat shock proteins from wheat

Small Hsps (sHsps) and the structurally related eye lens α‐crystallins are ubiquitous stress proteins that exhibit ATP‐independent molecular chaperone activity. We studied the chaperone activity of dodecameric wheat Ta Hsp16.9C‐I, a class I cytosolic sHsp from plants and the only eukaryotic sHsp for which a high resolution structure is available, along with the related wheat protein Ta Hsp17.8C‐II, which represents the evolutionarily distinct class II plant cytosolic sHsps. Despite the available structural information on Ta Hsp16.9C‐I, there is minimal data on its chaperone activity, and likewise, data on activity of the class II proteins is very limited. We prepared purified, recombinant Ta Hsp16.9C‐I and Ta Hsp17.8C‐II and find that the class II protein comprises a smaller oligomer than the dodecameric Ta Hsp16.9C‐I, suggesting class II proteins have a distinct mode of oligomer assembly as compared to the class I proteins. Using malate dehydrogenase as a substrate, Ta Hsp16.9C‐I was shown to be a more effective chaperone than Ta Hsp17.8C‐II in preventing heat‐induced malate dehydrogenase aggregation. As observed by EM, morphology of sHsp/substrate complexes depended on the sHsp used and on the ratio of sHsp to substrate. Surprisingly, heat‐denaturing firefly luciferase did not interact significantly with Ta Hsp16.9C‐I, although it was fully protected by Ta Hsp17.8C‐II. In total the data indicate sHsps show substrate specificity and suggest that N‐terminal residues contribute to substrate interactions.