Detection of a Compact Folding Intermediate of Dimethyl Sulfoxide Reductase Secreted from a Molybdenum Cofactor-Deficient Mutant ofRhodobacter sphaeroidesf. sp. denitrificans

All of the nine cysteine residues in dimethyl sulfoxide reductase (OMSOR) exist in reduced thiol form. The unfolded form, which was previously detected in DMSOR proteins secreted by spheroplasts prepared from a molybdenum cofactor-deficient mutant, was also detected in spheroplasts from a wild type strain when iodoacetamide was present, suggesting that DMSOR is secreted first in a reduced and unfolded form. In spheroplasts from the mutant, a new folding intermediate migrating between the unfolded and native forms was additionally detected on non-denaturing gel. This intermediate contained no disulfide bonds, but had a folded compact conformation similar to that of the native form.