Dynamics of small heat shock protein distribution within the chloroplasts of higher plants

Accumulation of the small heat shock proteins (sHSPs) in response to high temperature stress is thought to contribute to the development of thermotolerance in eukaryotic organisms, but the mechanism of action is unknown. We are investigating the chloroplast-localized sHSP, HSP21, with the goal of defining its contribution to the acquisition of thermotolerance in plants. Following an initial heat stress and period of recovery, HSP21 is localized primarily in the soluble fraction of the chloroplast. During an additional stress, HSP21 undergoes a temperature-dependent redistribution from the soluble to the insoluble chloroplast fraction in both isolated organelles and intact plants. The change in HSP21 partitioning is accompanied by depletion of the 10-11 S HSP21-containing complexes from the soluble chloroplast fraction. HSP21 in the insoluble fraction cannot be solubilized by nonionic detergent under conditions that release essentially all the pigments and proteins from the thylakoid membranes, indicating that HSP21 in its insoluble state is not dependent for its insolubility on attachment to an intact membrane. The temperature-dependent redistribution of HSP21 is affected by light intensity but occurs in both leaf and root plastids, suggesting that the function of this activity is not strictly related to the presence of the photosynthetic apparatus. Our study indicates that the chloroplast sHSP has dynamic properties similar to those of cytoplasmic sHSPs from plants and other organisms and suggests that the ability to partition between a soluble and an insoluble state reflects a functionally important property of all sHSPs.