Self-subunit Swapping Chaperone Needed for the Maturation of Multimeric Metalloenzyme Nitrile Hydratase by a Subunit Exchange Mechanism Also Carries Out the Oxidation of the Metal Ligand Cysteine Residues and Insertion of Cobalt

Abstract

The incorporation of cobalt into low molecular mass nitrile hydratase (L-NHase) of Rhodococcus rhodochrous J1 has been found to depend on the alpha-subunit exchange between cobalt-free L-NHase (apo-L-NHase lacking oxidized cysteine residues) and its cobalt-containing mediator (holo-NhlAE containing Cys-SO(2)(-) and Cys-SO(-) metal ligands), this novel mode of post-translational maturation having been named self-subunit swapping, and NhlE having been recognized as a self-subunit swapping chaperone (Zhou, Z., Hashimoto, Y., Shiraki, K., and Kobayashi, M. (2008) Proc. Natl. Acad. Sci. U. S. A. 105, 14849-14854). We discovered here that cobalt was inserted into both the cobalt-free NhlAE (apo-NhlAE) and the cobalt-free alpha-subunit (apo-alpha-subunit) in an NhlE-dependent manner in the presence of cobalt and dithiothreitol in vitro. Matrix-assisted laser desorption ionization time-of-flight mass spectroscopy analysis revealed that the non-oxidized cysteine residues in apo-NhlAE were post-translationally oxidized after cobalt insertion. These findings suggested that NhlE has two activities, i.e. cobalt insertion and cysteine oxidation. NhlE not only functions as a self-subunit swapping chaperone but also a metallochaperone that includes a redox function. Cobalt insertion and cysteine oxidation occurred under both aerobic and anaerobic conditions when Co(3+) was used as a cobalt donor, suggesting that the oxygen atoms in the oxidized cysteines were derived from water molecules but not from dissolved oxygen. Additionally, we isolated apo-NhlAE after the self-subunit swapping event and found that it was recycled for cobalt transfer into L-NHase.