The small heat shock proteins (sHsps), which counteract
heat and oxidative stress in an unknown way, belong to
a protein family of sHsps and α-crystallins whose members
form large oligomeric complexes. The chloroplast-localized
sHsp, Hsp21, contains a conserved methionine-rich sequence,
predicted to form an amphipatic helix with the methionines
situated along one of its sides. Here, we report how methionine
sulfoxidation was detected by mass spectrometry in proteolytically
cleaved peptides that were produced from recombinant Arabidopsis
thaliana Hsp21, which had been treated with varying
concentrations of hydrogen peroxide. Sulfoxidation of the
methionine residues in the conserved amphipatic helix coincided
with a significant conformational change in the Hsp21 protein
oligomer.