Heat-induced chaperone activity of serine/threonine protein phosphatase 5 enhances thermotolerance in Arabidopsis thaliana

• This study reports that Arabidopsis thaliana protein serine/threonine phosphatase 5 (AtPP5) plays a pivotal role in heat stress resistance. A high-molecular-weight (HMW) form of AtPP5 was isolated from heat-treated A. thaliana suspension cells. AtPP5 performs multiple functions, acting as a protein phosphatase, foldase chaperone, and holdase chaperone. The enzymatic activities of this versatile protein are closely associated with its oligomeric status, ranging from low oligomeric protein species to HMW complexes. • The phosphatase and foldase chaperone functions of AtPP5 are associated primarily with the low-molecular-weight (LMW) form, whereas the HMW form exhibits holdase chaperone activity. Transgenic over-expression of AtPP5 conferred enhanced heat shock resistance to wild-type A. thaliana and a T-DNA insertion knock-out mutant was defective in acquired thermotolerance. A recombinant phosphatase mutant (H290N) showed markedly increased holdase chaperone activity. • In addition, enhanced thermotolerance was observed in transgenic plants over-expressing H290N, which suggests that the holdase chaperone activity of AtPP5 is primarily responsible for AtPP5-mediated thermotolerance. • Collectively, the results from this study provide the first evidence that AtPP5 performs multiple enzymatic activities that are mediated by conformational changes induced by heat-shock stress.