Binding kinetics of histone chaperone Chz1 and variant histone H2A.Z-H2B by relaxation dispersion NMR spectroscopy.

The genome of eukaryotic cells is packed into a compact structure called chromatin that consists of DNA as well as histone and non-histone proteins. Histone chaperones associate with histone proteins and play important roles in the assembly of chromatin structure and transport of histones in the cell. The recently discovered histone chaperone Chz1 associates with the variant histone H2A.Z of budding yeast and plays a critical role in the exchange of the canonical histone pair H2A-H2B for the variant H2A.ZH2B. Here, we present an NMR approach that provides accurate estimates for the rates of association and dissociation of Chz1 and H2A.Z-H2B. The methodology exploits the fact that in a 1:1 mixture of Chz1 and H2A.Z-H2B, the small amounts of unbound proteins that are invisible in spectra produce line broadening of signals from the complex that can be quantified in terms of the thermodynamics and kinetics of the exchange process. The dissociation rate constant measured, 22 +/- 2 s(-1), provides an upper bound for the rate of transfer of H2A.Z-H2B to the chromatin remodeling complex, and the faster-than-diffusion association rate, 10(8) +/- 10(7) M(-1) s(-1), establishes the importance of attractive electrostatic interactions that form the chaperone-histone complex.