Concentration Dependent Ion–Protein Interaction Patterns Underlying Protein Oligomerization Behaviours

Abstract

Most proteins do not exist as monomers. Instead, proteins assemble into oligomeric structures, which range from small dimers to intermediately sized clusters to large polymers. Oligomerization is driven by protein–protein interactions between charged residues, (induced) dipoles, aromatic residues and hydrophobic patches. <br /> Ionic protein–protein interactions, and thus the oligomeric state of a protein, can be influenced by metal ions. There are several theories that strive to explain interactions between metal ions and charged proteins. Continuum electrostatic theories assume a decaying electrostatic potential from a charged protein surface which attracts oppositely charged ions to the point of charge neutralization, while the water solvent is treated as passive medium characterized only by its permittivity. More recent concepts, however, recognize the importance of water coordination. The hydration enthalpy of metal ions and ionic protein groups is envisaged as the driving force for ion pairing. <br /> Research and theory have so far focussed on single protein species in simple aqueous solutions. This work comparatively analyses Ca²⁺-induced oligomerization of the negatively charged SNAP25 protein in solution and in the crowded multi-component environment of the plasma membrane. It proves ion-induced protein oligomerization to be a fundamental chemico-physical principle that is conserved in both environments. The restricted protein movement and the manifold interactions with other proteins and lipids in the membrane appear to mainly influence the number of monomers comprised in an oligomer, but not the phenomenon of oligomerization itself. <br /> Comparison of Ca²⁺ to other positively charged metal ions indicates that ions need to convey a certain charge density and to possess a certain water affinity to induce membrane protein clustering. The results suggest a direct interaction between calcium ions and negatively charged protein residues. It appears that the stoichiometry of calcium–carboxylate group interactions determines the degree of oligomerization. At low calcium concentrations which induce protein clustering, the ions function as bridges between the carboxylate groups, and attenuate the negative protein charge and thus repulsive protein–protein interactions. At high calcium concentrations, binding of one or more calcium ions to a single negatively charged residue is frequently encountered. The calcium ions thus no longer function as bridges between several carboxylate groups. In addition, the local overcharging entails repulsive forces between proteins which again favour protein dispersion. The study provides a conceptual framework for the influence of ions on electrostatically driven protein–protein interactions and protein aggregation with implications for biological and industrial settings.