Osmotic pressure (Ψ_π) is a highly complementary physical parameter to hydrostatic pressure (Ψ_p) for understanding the hydration properties of biomolecular systems. While Ψ_p probes volume changes of the system as a whole, Ψ_π samples hydration more directly via the linked interactions between the biological solute and osmolyte. Practically, interactions in solution are sensitive to kbar and higher levels of hydrostatic pressure, as is well established in the high pressure literature, and is directly relevant to extremophiles living under hyperbaric conditions in the deep sea. In contrast, even several bars of osmotic pressure (Ψ_π) are severely perturbative in all organisms depending on the hydration properties of the interaction at hand. Thus, intracellular Ψ_π is a primary parameter for correlating hydration to functional biology in vivo.

            Our group has been employing “high” osmotic pressure extensively to understand the basis of site-specific DNA interactions by proteins^{1, 2, 3, 4} and drug-like ligands⁵ in solution, using co-solutes that are strongly excluded from macromolecular surfaces (“compatible osmolytes” such as betaines). We have found that hydration represents a mechanism by which structurally homologous DNA-binding domains evolve diversity and confer sensitivity to the osmotic environment, as well as to tune DNA sequence selectivity in the absence of osmotic stress. A fruitful model system has been the ETS-family of transcription factors, an ancient class of gene regulators that are ubiquitous in the animal kingdom. Currently a major effort is to elaborate the structure and dynamics of interfacial hydration through experimental measurements of the binding thermodynamics and explicit-solvent simulations of protein/DNA systems. Ultimately, our goal is to understand the how the transcription machinery, a major component of cellular responsiveness, is physically coupled to it the osmotic environment.

REFERENCES

1.   Albrecht AV, Kim HM, Poon GMK. Mapping interfacial hydration in ETS-family transcription factor complexes with DNA: a chimeric approach. Nucleic Acids Res 46, 10577-10588 (2018).

2.   Xhani S, Esaki S, Huang K, Erlitzki N, Poon GM. Distinct Roles for Interfacial Hydration in Site-Specific DNA Recognition by ETS-Family Transcription Factors. J Phys Chem B 121, 2748-2758 (2017).

3.   Wang S, Linde MH, Munde M, Carvalho VD, Wilson WD, Poon GM. Mechanistic heterogeneity in site recognition by the structurally homologous DNA-binding domains of the ETS family transcription factors Ets-1 and PU.1. J Biol Chem 289, 21605-21616 (2014).

4.   Poon GM. Sequence discrimination by DNA-binding domain of ETS family transcription factor PU.1 is linked to specific hydration of protein-DNA interface. J Biol Chem 287, 18297-18307 (2012).

5.   Erlitzki N, et al. Investigation of the electrostatic and hydration properties of DNA minor groove-binding by a heterocyclic diamidine by osmotic pressure. Biophys Chem 231, 95-104 (2017).