Recent experimental studies show that oppositely charged proteins can self-assemble to form seemingly stable microspheres in aqueous salt solutions. We here use parallel tempering Monte Carlo simulations to study protein phase separation of lysozyme/α-lactalbumin mixtures and show that anisotropic electrostatic interactions are important for driving protein self-assembly. In both dilute and concentrated protein phases, the proteins strongly align according to their charge distribution. While this alignment can be greatly diminished by a single point mutation, phase separation is completely suppressed when neglecting electrostatic anisotropy. The results highlight the importance of subtle electrostatic interactions even in crowded biomolecular environments where other short-ranged forces are often thought to dominate.

Histidine rich, unstructured peptides adsorb to charged interfaces such as mineral surfaces and microbial cell membranes. At a molecular level, we investigate the adsorption mechanism as a function of pH, salt, and multivalent ions showing that (1) proton charge fluctuations are -in contrast to the majority of proteins -optimal at neutral pH, promoting electrostatic interactions with anionic surfaces through charge regulation, and (2) specific zinc(II)-histidine binding competes with protons and ensures an unusually constant charge distribution over a broad pH interval. In turn this further enhances surface adsorption. Our analysis is based on atomistic molecular dynamics simulations, coarse grained Metropolis Monte Carlo, and classical polymer density functional theory. This multi-scale modelling provides a consistent picture in good agreement with experimental data on Histatin 5, an antimicrobial salivary peptide. Biological function is discussed and we suggest that charge regulation is a significant driving force for the remarkably robust activity of histidine rich antimicrobial peptides.

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Oblivious to ion specificity, pH has been a key parameter for macromolecular solutions for little more than a century. We here widen the concept by describing the ionization of macromolecules not only via pH, but also pX where X are other binding species. Using binding constants, measured by NMR, of chloride and thiocyanate to amino acid motifs on g-crystallin, we calculate i) titration curves as a function of pH and pX and ii) estimate second virial coefficients using both approximate theory and computer simulations. In agreement with experiment, a Hofmeister reversal for protein-protein interactions is observed when crossing iso-electric conditions. Thiocyanate binding further leads to large charge fluctuations that may trigger intermolecular charge regulation interactions.

Mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene result in impaired host defense during cystic fibrosis (CF), where Pseudomonas aeruginosa becomes a key pathogen. We investigated the expression pattern of the antibacterial growth factor midkine (MK) in CF and the possible interference with its activity by the altered airway microenvironment. High MK expression was found in CF lung tissue compared with control samples, involving epithelia of the large and small airways, alveoli, and cells of the submucosa (i.e., neutrophils and mast cells). In CF sputum, MK was present at 100-fold higher levels, but was also subject to increased degradation, compared with MK in sputum from healthy control subjects. MK exerted a bactericidal effect on P. aeruginosa, but increasing salt concentrations and low pH impaired this activity. Molecular modeling suggested that the effects of salt and pH were attributable to electrostatic screening and a charge-neutralization of the membrane, respectively. Both the neutrophil elastase and elastase of P. aeruginosa cleaved MK to smaller fragments, resulting in impaired bactericidal activity. Thus, MK is highly expressed in CF, but its bactericidal properties may be impaired by the altered microenvironment, as reflected by the in vitro conditions used in this study.