N-terminal arm of orchardgrass Hsp17.2 (DgHsp17.2) is essential for both in vitro chaperone activity and in vivo thermotolerance in yeast

Small heat shock proteins are well-known to function as chaperone in the protection of proteins and subcellular structures against stress-induced denaturation in many cell compartments. Irrespective of such general functional assignment, a proof of function in a living organism is missing. Here, we used heat-induced orchardgrass small Hsp17.2 (DgHsp17.2). Its function in in vitro chaperone properties has shown in protecting the model substrate, malate dehydrogenase (MDH) and citrate synthase (CS). Overexpression of DgHsp17.2 triggering strong chaperone activity enhanced in vivo thermotolerance of yeast cells. To identify the functional domain on DgHsp17.2 and correlationship between in vitro chaperone property and in vivo thermotolerance, we generated truncation mutants of DgHsp17.2 and showed essentiality of the N-terminal arm of DgHsp17.2 for the chaperone function. In addition, beyond for acquisition of thermotolerance irrespective of sequences are diverse among the small Hsps. However, any truncation mutants of DgHsp17.2 did not exhibit strong interaction with orchardgrass heat shock protein 70 (DgHsp70) different from mature DgHsp17.2, indicating that full-length DgHsp17.2 is necessary for cooperating with Hsp70 protein. Our study indicates that the N-terminal arm of DgHsp17.2 is an important region for chaperone activity and thermotolerance.