Binding Affinity and Function of the Extremely Disordered Protein Complex Containing Human Linker Histone H1.0 and Its Chaperone ProT alpha

It was recently reported that human linker histone H1.0 and its chaperone prothyrnosin-alpha (ProT alpha) form an extremely disordered 1:1 complex with an ultrahigh affinity (equilibrium dissociation constant K-D of similar to 2 x 10(-12) M) measured using a single-molecule Forster resonance energy transfer method. It was hypothesized that the ultrahigh affinity and extreme disorder may be required for the chaperone function of ProT alpha, in which it displaces the linker histone from condensed chromatin. Here, we measure the binding affinity for the ProT alpha-H1.0 complex using isothermal titration calorimetry and report a K-D value of (4.6 +/- 0.5) x 10(-7) M. In addition, we show that ProT alpha facilitates the formation of the H1.0-nucleosome complex in vitro. The results of our study contrast with those of the previous report and provide new insights into the chaperone function of ProT alpha. Possible causes for the observed discrepancy in binding affinity are discussed.