Chaperone activity of cytosolic small heat shock proteins from wheat

Small Hsps (sHsps) and the structurally related eye lens alpha-crystallins are ubiquitous stress proteins that exhibit ATP-independent molecular chaperone activity. We studied the chaperone activity of dodecameric wheat TaHsp16.9C-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 TaHsp17.8C-II, which represents the evolutionarily distinct class II plant cytosolic sHsps. Despite the available structural information on TaHsp16.9C-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 TaHsp16.9C-I and TaHsp17.8C-II and find that the class II protein comprises a smaller oligomer than the dodecameric TaHsp16.9C-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, TaHsp16.9C-I was shown to be a more effective chaperone than TaHsp17.8C-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 TaHsp16.9C-I, although it was fully protected by TaHsp17.8C-II. In total the data indicate sHsps show substrate specificity and suggest that N-terminal residues contribute to substrate interactions.