Abstract
Small heat shock proteins (sHSPs) have been thought to function as chaperones, protecting their targets from denaturation and aggregation when organisms are subjected to various biotic and abiotic stresses. We previously reported an sHSP from Oryza sativa (OsHSP20) that homodimerizes and forms granules within the cytoplasm but its function was unclear. We now show that OsHSP20 transcripts were significantly up-regulated by heat shock and high salinity but not by drought. A recombinant protein was purified and shown to inhibit the thermal aggregation of the mitochondrial malate dehydrogenase (MDH) enzyme in vitro, and this molecular chaperone activity suggested that OsHSP20 might be involved in stress resistance. Heterologous expression of OsHSP20 in Escherichia coli or Pichia pastoris cells enhanced heat and salt stress tolerance when compared with the control cultures. Transgenic rice plants constitutively overexpressing OsHSP20 and exposed to heat and salt treatments had longer roots and higher germination rates than those of control plants. A series of assays using its truncated mutants showed that its N-terminal arm plus the ACD domain was crucial for its homodimerization, molecular chaperone activity in vitro, and stress tolerance in vivo. The results supported the viewpoint that OsHSP20 could confer heat and salt tolerance by its molecular chaperone activity in different organisms and also provided a more thorough characterization of HSP20-mediated stress tolerance in O. sativa.
Highlights
Small heat shock proteins have been thought to function as chaperones, protecting their targets from denaturation and aggregation when organisms are subjected to various biotic and abiotic stresses
OsHSP20 mRNAs were significantly more abundant in the leaves of stressed plants compared to the controls at 2, 4, 8 and 12 h and peaked with values 14–18 fold higher at 24–36 h (Fig. 1B)
The results suggested that the N-terminal arm plus the alpha-crystallin domain (ACD) domain was necessary for the homodimerization of OsHSP20 and for its in vitro chaperone and stress tolerance activity
Summary
Small heat shock proteins (sHSPs) have been thought to function as chaperones, protecting their targets from denaturation and aggregation when organisms are subjected to various biotic and abiotic stresses. Heat shock proteins (HSPs) have been thought to function as molecular chaperones, which are ubiquitous and evolutionarily conserved in both animals and plants and protect their target proteins from denaturation, misfolding and aggregation when subjected to various stresses in organisms[1,2,3,4,5,6,7]. Under biotic and abiotic stress conditions, a wide range of plant sHSPs have been revealed to act as molecular chaperones in vitro and in vivo independent of ATP, preventing protein denaturation or aggregation[9,25,26,27,28]. The key domains in the protein crucial for its homodimerization, molecular chaperone activity in vitro, and stress tolerance were further determined using a series of truncated mutants
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