Mycobacterium tuberculosis (TB) small heat shock protein Hsp16.3 was found to be a major membrane protein that is most predominantly expressed under oxidative stress and is localized to the thickened cell envelope. Gene knock-out studies indicate that the Hsp16.3 protein is required for TB to grow in its host macrophage cells. The physiological function of Hsp16.3 has not yet revealed. Our analyses via mass spectrometry, conformation-dependent trypsin digestion, nondenaturing pore gradient electrophoresis, ANS-binding fluorescence measurements, and circular dichroism demonstrate that the three and only the three methionine residues (cysteine and tryptophan residues, which can also be readily oxidized by such oxidant as H2O2, are absent in Hsp16.3) can be readily sulfoxidized with H2O2 treatment in vitro, and the methionine sulfoxide can be effectively reduced back to the methionine form. Interconversion between the methionine and methioninesulfoxide has been confirmed by selective oxidation and reduction. The sulfoxidation leads to a small degree of conformational change, which in turn results in a significant decrease of the chaperone-like activity. Data presented in this report strongly implicate that reversible sulfoxidation/desulfoxidation of methionine residues may occur in Hsp16.3, which serves as a way to scavenger reactive oxygen or nitrogen species abundantly present in macrophage cells, thus protecting the plasma membrane and other components of M. tuberculosis allowing their survival in such bacteriocidal hosts. (C) 2003 Elsevier Science (USA). All rights reserved.