The binding affinity and function of the extremely disordered protein complex containing human linker histone H1.0 and its chaperone ProTα.

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