Binding between particles and proteins in extracts: implications for microrheology and toxicity

Understanding and controlling the interactions between foreign materials and cytoplasmic proteins is key for the design of intracellular probes, and for uncovering mechanisms of micro and nanoparticle toxicity. Here we examine these interactions by characterizing protein adsorption from cell extracts to a range of micron and sub-micron particles, and by measuring the Brownian motions of particles in live cells and reconstituted networks as an in situ measure of association. Testing SiO2, TiO2 and polystyrene particles with varying surface carboxylation, together with protein and polyethylene glycol surface coatings, we find that cellular associations and protein binding both strongly depend on particle surface chemistry. Cytoskeletal proteins, most notably actin and intermediate filament family members, are among the proteins most concentrated on the surfaces of all particles tested. The nanoscale movements of microinjected particles that primarily bind vimentin intermediate filaments are larger than particles that can also bind actin. This difference disappears when the same particles are endocytosed, suggesting that endocytic membranes mask particle surfaces. We discovered one brand of carboxylated SiO2 particles that is remarkably resistant to protein binding in extracts. By coupling the actin binding molecule phalloidin to these particles, we converted their surface from non-binding to actin-binding. We illustrate the efficacy of the conversion in reconstituted actin gels.