Minimal structural perturbation of Histatin 5 in crowded environments: Insights from small-angle X-ray scattering, dynamic light scattering, and computer simulations.

The structure and dynamics of intrinsically disordered proteins (IDPs) are malleable to solution conditions. Considerable effort has been devoted to understanding the effects of molecular crowding on these properties. Polymer-based crowders, such as polyethylene glycol (PEG) and Ficoll®, are commonly used to simulate the intracellular environment by replicating the high concentration of macromolecules. In this study, we examine the impact of crowding on the IDP Histatin 5 using PEG of various molecular weights and Ficoll® PM 70. Small-angle X-ray scattering (SAXS) reveals minimal effects on the structural ensemble of Histatin 5 in the presence of both PEG and Ficoll® PM 70. These findings are further supported by dynamic light scattering (DLS) experiments, which confirm the SAXS results. However, as the molecular weight of PEG increases, the concentration at which the semidilute regime is reached decreases. A similar trend is observed for the larger crowder, Ficoll® PM 70. Computational models align with the experimental results, suggesting negligible crowding effects. Additionally, simulations indicate that the crowders undergo greater conformational changes with increasing concentration than Histatin 5. These findings support the idea that smaller IDPs, such as Histatin 5, could exhibit a rigid, rod-like conformation that makes them resistant to crowding effects.