Immunological kinship of class I low molecular weight heat shock proteins and thermostabilization of soluble proteins in vitro among plants

The antibody prepared against one of the soybean (Glycine max) 15 to 18 kDa heat shock proteins (HSPs) that cross-reacted with the 12 polypeptides of 15 to 18 kDa class I low molecular weight (LMW) HSPs in soybean, was also found to cross-react in Western blot analyses with the class I LMW HSPs of nine other plant species, I.e., mung bean, pea, cucumber, tobacco, Arabidopsis, rice, maize, wheat, and barley. An antibody raised from the 16.9 kDa rice HSP also crossreacted with the same class I LMW HSPs of the ten plant species tested. HSPs-enriched fractions (70 to 100% ammonium sulfate saturation) prepared from mung bean and rice heat-shocked seedlings were able to thermostabilize the homologous soluble proteins, as we have shown previously in soybean. Up to 50% of the soluble proteins that are normally denatured by heating at 55°C for 30 min was protected when an HSPs-enriched fraction was added to either mung bean or rice protein. Additionally, the HSPs-enriched fractions were exchangeable among these three plant species for thermostabilization. The protection provided by these HSPs-enriched fractions is effective mainly for membrane-associated proteins. In soybean depletion of the 15 to 18 kDa HSPs in the HSPs-enriched fraction resulted in the loss of the thermostabilizing ability and when the 15 to 18 kDa HSPs were recovered in this fraction, the thermostabilizing ability was again restored. Thus, the 15 to 18 kDa HSPs in plant, which shuttle between the cytoplasm and cellular organelles during heat shock (HS) and recovery from HS, are responsible for providing the thermostabilization.