UreE stimulation of GTP-dependent urease activation in the UreD-UreF-UreG-urease apoprotein complex

The activation of metal-containing enzymes often requires the participation of accessory proteins whose roles are poorly understood. In the case of Kl ebsiella aerogenes urease, a nickel-containing enzyme, metallocenter assemb ly requires UreD, UreF, and UreG acting as a protein chaperone complex and UreE serving as a nickel metallochaperone. Urease apoprotein within the Ure D-UreF-UreG-urease apoprotein complex is activated to wild-type enzyme acti vity levels under physiologically relevant conditions (100 mu M bicarbonate and 20 mu M Ni2+) in a process that requires GTP and UreE. The GTP concent ration needed for optimal activation is greatly reduced in the presence of UreE compared to that required in its absence. The amount of UreE provided is critical, with maximal activation observed at a concentration equal to t hat of Ni2+. On the basis of its ability to facilitate urease activation in the presence of chelators, UreE is proposed to play an active role in tran sferring Ni2+ to urease apoprotein. Studies involving site-directed variant s of UreE provide evidence that His96 has a direct role in metal transfer. The results presented here parallel those obtained from previous in vivo st udies, demonstrating the relevance of this in vitro system to the cellular metallocenter assembly process.