Escherichia coli DmsD interacts with the twin-arginine leader sequence of the catalytic sub-unit (DmsA) of DMSO reductase. DmsD was purified as a mixture of a number of different folding forms including: dimer (A); monomer (13); a minor thiol oxidized form; a heterogeneously folded or multi-conformational monomer form which displayed a ladder of bands on native-PAGE (D); and proteolytically degraded and aggregated forms. Polyacrylamide gel electrophoresis (PAGE), under denaturing and non-denaturing conditions, was used to examine the folding and stability of DmsD. Additionally, the biophysical methods of dynamic light scattering, circular dichroism, fluorescence, and mass spectroscopy were also used. Form D could be converted to form B by treatment with 4 M urea, which is the concentration at which form B begins to denature. Forms A/B could be converted to D by incubation at pH 5.0. Forms A/B and D all had twin-arginine leader binding activity. (C) 2004 Elsevier Inc. All rights reserved.