Interplay between Heat Shock Proteins, HSP101 and HSA32, Prolongs Heat Acclimation Memory Posttranscriptionally in Arabidopsis
Heat acclimation improves the tolerance of organisms to severe heat stress. Our previous work showed that in Arabidopsis the “memory” of heat acclimation treatment decayed faster in the absence of HSA32, a heat-induced protein predominantly found in plants. HSA32 null mutant attains normal short-term acquired thermotolerance (SAT), but is defective in long-term acquired thermotolerance (LAT). To further explore this phenomenon, we isolated Arabidopsis dlt (defective in lat) mutants using a forward genetic screen. Two recessive missense alleles, dlt1-1 and dlt1-2, encode the molecular chaperone HSP101. Results of immunoblot analyses suggest that HSP101 enhances the translation of HSA32 during recovery after heat treatment, and in turn HSA32 retards the decay of HSP101. dlt1-1 mutation has little effect on HSP101 chaperone activity and thermotolerance function, but compromises the regulation of HSA32. In contrast, dlt1-2 impairs chaperone activity and thermotolerance function of HSP101, but not on the regulation of HSA32. These results suggest that HSP101 has a dual function, which could be decoupled by the mutations. Pulse-chase analysis showed that HSP101 degraded faster in the absence of HSA32. Autophagic proteolysis inhibitor E-64d, but not the proteasome inhibitor MG132, inhibited the degradation of HSP101. Ectopic expression of HSA32 confirmed its effect on the decay of HSP101 at posttranscriptional level, and showed that HSA32 was not sufficient to confer LAT when the HSP101 level was low. Taken together, our results indicate a novel mechanism for prolonging the memory of heat acclimation modulated by a positive feedback loop between HSP101 and HSA32.
