Zinc binding of Tim10: Evidence for existence of an unstructured binding intermediate for a zinc finger protein

Zinc-finger proteins are among the most abundant proteins in eukaryotic genomes. Tim10 and all the small Tim proteins of the mitochondrial intermembrane space contain a consensus twin CX3C zinc-finger motif. Zn 2+ can bind to the reduced Tim10, but not disulphide bonded (oxidized) protein. However, the zinc-binding reaction of Tim10 and of zinc-finger proteins, in general, is ill-defined. In this study, the thermodynamic and kinetic properties of zinc-binding to reduced Tim10 were investigated using circular dichroism (CD), fluorescence spectrometry, and stopped-flow fluorescence techniques. At equilibrium, coupled with the use of protein fluorescence and metal chelators, the zinc-binding affinity was determined for Tim10 to be about 8 × 10-10M. Then, far UV CD was used to investigate the secondary structure change upon zinc-binding of the same set of protein samples at various free Zn2+ concentrations. Comparison between the results of CD and fluorescence studies showed that the zinc-binding reaction is not a simple one-step process. It involves formation of a binding intermediate that is structurally as unfolded as the apoTim10; subsequently, a degree of folding is induced at increased zinc concentrations in the final complex. Next, the stopped-flow fluorescence technique was used to investigate the kinetic process of the binding reaction. Data analysis shows that the reaction has a single kinetic phase at a low free Zn2+ concentration (∼1 nM), and a double kinetic phase at a high free Zn 2+ concentration. The kinetic result is consistent with that of the studies at equilibrium. Therefore, a two-step reaction model mechanism is proposed, in which zinc-binding is regulated by the initial selective-binding of Zn2+ to Cys followed by folding. Implication of the two-step zinc-binding mechanism for Zn2+ trafficking in the cell is discussed. © 2007 Wiley-Liss, Inc.