Highly charged intrinsically disordered proteins can form complexes with very high affinity in which both binding partners fully retain their disorder and dynamics, exemplified by the positively charged linker histone H1.0 and its chaperone, the negatively charged prothymosin α. Their interaction exhibits another surprising feature: The association/dissociation kinetics switch from slow two-state-like exchange at low protein concentrations to fast exchange at higher, physiologically relevant concentrations. Here we show that this change in mechanism can be explained by the formation of transient ternary complexes favored at high protein concentrations that accelerate the exchange between bound and unbound populations by orders of magnitude. Molecular simulations show how the extreme disorder in such polyelectrolyte complexes facilitates (i) diffusion-limited binding, (ii) transient ternary complex formation, and (iii) fast exchange of monomers by competitive substitution, which together enable rapid kinetics. Biological polyelectrolytes thus have the potential to keep regulatory networks highly responsive even for interactions with extremely high affinities.
Bibliographical noteFunding Information:
We thank Erik Holmstrom, Louise Pinet, Andreas Prestel, and Chris Waudby for helpful discussions, Iwo König and Jacob Hertz Martinsen for protein production, Jendrik Schöppe and Andreas Plückthun for the pAT222-pD expression vector, and the Functional Genomics Center Zurich for performing mass spectrometry. This work was supported by the Swiss National Science Foundation (to B.S.), the Forschungskredit of the University of Zurich (to A.S., FK-17-038, and A.C., FK-19-039), the Novo Nordisk Foundation Challenge program REPIN (NNF18OC0033926 to B.B.K. and B.S.), and the Intramural Research Program of the NIDDK at the National Institutes of Health (R.B.B.). This work utilized the computational resources of the NIH HPC Biowulf cluster (http:// hpc.nih.gov).
© 2020, The Author(s).