Interplay between Heat Shock Proteins HSP101 and HSA32 Prolongs Heat Acclimation Memory Posttranscriptionally in Arabidopsis    

Abstract

Heat acclimation improves the tolerance of organisms to severe heat stress. Our previous work showed that in Arabidopsis (Arabidopsis thaliana), the "memory" of heat acclimation treatment decayed faster in the absence of the heat-stress-associated 32-kD protein HSA32, a heat-induced protein predominantly found in plants. The HSA32 null mutant attains normal short-term acquired thermotolerance but is defective in long-term acquired thermotolerance. To further explore this phenomenon, we isolated Arabidopsis defective in long-term acquired thermotolerance (dlt) mutants using a forward genetic screen. Two recessive missense alleles, dlt1-1 and dlt1-2, encode the molecular chaperone heat shock protein101 (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. The dlt1-1 mutation has little effect on HSP101 chaperone activity and thermotolerance function but compromises the regulation of HSA32. In contrast, dlt1-2 impairs the chaperone activity and thermotolerance function of HSP101 but not 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. The 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 the posttranscriptional level and showed that HSA32 was not sufficient to confer long-term acquired thermotolerance when the HSP101 level was low. Taken together, we propose that a positive feedback loop between HSP101 and HSA32 at the protein level is a novel mechanism for prolonging the memory of heat acclimation.

Interaction Studies of Heat Shock Responsive Protein HSA32 and Heat Shock Protein HSP101: An In Silico Approach

“…Hsa32 is a good candidate for this purpose. It is observed that Hsa32 was upregulated by heat shock treatment in plants [2]. It shows its important role in HSR during plant evolution.…”

“…It suggests a role of regulating HSP101 decay in the adaptation to changing climates. Very recently, using rice Tos17 retrotransposon insertion lines, it is observed that disruption of HSA32 also led to faster decay of HSP101 in Arabidopsis [2]. Future investigations on the structure-function association of HSA32 and HSP101 in plant species will provide us more insight into the high temperature tolerance mechanism.…”

A Positive Feedback Loop between HEAT SHOCK PROTEIN101 and HEAT STRESS-ASSOCIATED 32-KD PROTEIN Modulates Long-Term Acquired Thermotolerance Illustrating Diverse Heat Stress Responses in Rice Varieties

“…In Arabidopsis, HSA32 was shown to retard the degradation of heat-induced HSP101 (Wu et al, 2013). To see whether rice HSA32 has similar function, HSP101 level was examined in HSA32 KO mutant and the wild-type seedlings during recovery after heat Figure 2.…”

“…In the HSA32 knockout (KO) or knockdown mutants in Arabidopsis, acquired thermotolerance was induced to wild-type level after a short recovery of 2 h following acclimation treatment at 37°C but was compromised significantly after a long recovery for 48 h. Based on this phenotype, acquired thermotolerance retained after a long recovery period was named longterm acquired thermotolerance (LAT), as opposed to the short-term acquired thermotolerance (SAT) that is attained after a short recovery (Yeh et al, 2012). Recently, it was shown that HSA32 regulates the degradation rate of HSP101, a cytosolic caseinolytic peptidase B (ClpB)/ HSP100 family protein in plants (Wu et al, 2013). Cytosolic ClpB/HSP100 proteins are molecular chaperones that promote the renaturation of protein aggregates (Mogk et al, 2008) and are required for the development of acquired thermotolerance (Sanchez and Lindquist, 1990;Hong and Vierling, 2000;Queitsch et al, 2000;Nieto-Sotelo et al, 2002).…”

“…In the absence of HSA32, heat-induced HSP101 is degraded faster. Interestingly, the level of HSA32 protein itself is positively regulated by HSP101, suggesting that HSA32 and HSP101 form a positive feedback loop to prolong the memory of heat acclimation (Wu et al, 2013).…”