CHARACTERIZATION OF DSBC, A PERIPLASMIC PROTEIN OF ERWINIA-CHRYSANTHEMI AND ESCHERICHIA-COLI WITH DISULFIDE-ISOMERASE ACTIVITY

We identified and characterized an Erwinia chrysanthemi gene able to c omplement an Escherichia coli dsbA mutation that prevents disulfide bo nd formation in periplasmic proteins. This gene, dsbC, codes for a 24 kDa periplasmic protein that contains a characteristic active site seq uence of disulfide isomerases, Phe-X-X-X-X-Cys-X-X-Cys. Besides the ac tive site, DsbC has no homology with DsbA, thioredoxin or eukaryotic p rotein disulfide isomerase and it could define a new subfamily of disu lfide isomerases. Purified DsbC protein is able to catalyse insulin ox idation in a dithiothreitol dependent manner. The E.coli gene xprA cod es for a protein functionally equivalent to DsbC. The in vivo function of DsbC seems to be the formation of disulfide bonds in proteins. The presence of XprA could explain the residual disulfide isomerase activ ity existing in dsbA mutants. Re-oxidation of XprA does not seem to oc cur through DsbB, the protein that probably re-oxidizes DsbA.