The chloroplast small heat shock protein undergoes oxidation-dependent conformational changes and may protect plants from oxidative stress

The nuclear-encoded chloroplast-localized Hsp21 is an oligomeric heat shock protein (Hsp), belonging to the protein family of small Hsps and alpha-cry stallins. We have investigated the effects of high temperature and oxidatio n treatments on the structural properties of Hsp21, both in purified recomb inant form and in transgenic Arabidopsis thaliana plants engineered to cons titutively overexpress Hsp21. A conformational change was observed for the 300 kDa oligomeric Hsp21 protein during moderate heat stress (less than or equal to 40 degrees C) of Arabidopsis plants, as judged by a shift to lower mobility in non-denaturing electrophoresis. Similar changes in mobility we re observed when purified recombinant Hsp21 protein was subjected to an oxi dant. Exposure of Hsp21 protein to temperatures above 70 degrees C led to i rreversible aggregation, which was prevented in presence of the reductant d ithiothreitol. The transgenic plants that constitutively overexpressed Hsp2 1 were more resistant to heat stress than were wildtype plants when the hea t stress was imposed under high light conditions. These results suggest tha t the physiological role of Hsp21 involves a response to temperature-depend ent oxidative stress.