New mechanism for post-translational processing during assembly of a cytoplasmic membrane protein?

Insertion of nitrate reductase into the Escherichia coli cytoplasmic membrane was examined by following the fate of pulse-labeled enzyme in both the membrane and cytoplasm during various times after the addition of an unlabeled chase. The polypeptide composition of this labeled enzyme was determined by autoradiography of immunoprecipitated material after separation on sodium dodecyl sulfate-polyacrylamide gels. The data presented here indicate that immediately after appropriate insertion of the enzyme into the membrane, a post-translational event occurs which converts the cytoplasmically synthesized form of subunit B (B') to the form found in the completely assembled enzyme (B). B' is distinguished from B by its more rapid electrophoretic mobility. B' was found in the cytoplasm of all strains tested, in the membrane of strains with defects in enzyme insertion (hemA and chlE), and as a transient component in the membrane of wild-type cells.