Methionine sulfoxidation of the chloroplast small heat shock protein and conformational changes in the oligomer

The small heat shock proteins (sHsps), which counteract heat and oxidative stress in an unknown way, belong to a protein family of sHsps and alpha-cry stallins whose members form large oligomeric complexes. The chloroplast-loc alized sHsp, Hsp21, contains a conserved methionine-rich sequencer predicte d to form an amphipatic helix with the methionines situated along one of it s sides. Here, we report how methionine sulfoxidation was detected by mass spectrometry in proteolytically cleaved peptides that were produced from re combinant Arabidopsis thaliana Hsp21, which had been treated with varying c oncentrations of hydrogen peroxide. Sulfoxidation of the methionine residue s in the conserved amphipatic helix coincided with a significant conformati onal change in the Hsp21 protein oligomer.