Metallochaperone function of the self-subunit swapping chaperone involved in the maturation of subunit-fused cobalt-type nitrile hydratase.

The transition metal (iron or cobalt) is a mandatory part that constitutes the catalytic center of nitrile hydratase (NHase). The incorporation of the cobalt ion into cobalt-containing NHase (Co-NHase) was reported to depend on self-subunit swapping and the activator of the Co-NHase acts as a self-subunit swapping chaperone for subunit exchange. Here we discovered that the activator acting as a metallochaperone transferred the cobalt ion into subunit-fused Co-NHase. We successfully isolated two activators, P14K and NhlE, which were the activators of NHases from Pseudomonas putida NRRL-18668 and the activator of low-molecular-mass NHase from Rhodococcus rhodochrous J1, respectively. Cobalt content determination demonstrated that NhlE and P14K were two cobalt-containing proteins. Substitution of the amino acids involved in the C-terminus of the activators affected the activity of the two NHases, indicating that the potential cobalt-binding sites might be located at the flexible C-terminal region. The cobalt-free NHases could be activated by either of the two activators, and both the two activators activated their cognate NHase more efficiently than did the noncognate ones. This study provided insights into the maturation of subunit-fused NHases and confirmed the metallochaperone function of the self-subunit swapping chaperone.