Revealing properties of the KfrA plasmid protein via combined DLS, AFM and electrokinetic measurements

Physicochemical characteristics of the plasmid KfrA protein in electrolyte solutions were done using a combination of dynamic light scattering (DLS), atomic force microscopy (AFM) and electrokinetic methods. The size of the protein was determined via the diffusion coefficient measurements using DLS. It was revealed from these measurements that the protein exists in an aggregated state composed of four molecules. The size of the protein was also determined via AFM imaging of single molecules adsorbed on mica from dilute solutions at pH = 3.5. It was 10.6 nm in accordance with the value predicted for an aggregate composed of four monomers in a hexagonal configuration. The aggregation number was also confirmed by kinetics measurements carried out under diffusion controlled transport using AFM imaging of proteins. Further characteristics were acquired via KfrA adsorption on polystyrene latex particles (average size of 820 nm). The electrophoretic mobility of the latex and its zeta potential were determined as a function of the coverage of the protein. The maximum monolayer coverage for pH = 3.5 was 1.2 mg m−2. Additionally, from these measurements the effective charge of KfrA tetramer equal to 12e (elementary charges) was predicted. The KfrA monolayer on latex was used to determine the isoelectric point of the protein, which was pH = 4.5. As concluded, the procedures used in our work proved advantageous for a direct determination of aggregation processes and the effective charge if minor amounts of a protein are available.