PROTEIN INTERACTIONS AS SEEN BY SOLUTION X-RAY-SCATTERING PRIOR TO CRYSTALLOGENESIS

In a previous work, solubility diagrams of lysozyme and its ability to crystallize in the presence of various ions were established. Inorgan ic as well as organic anions showed strong effects on lysozyme solubil ity following the reverse order of the Hofmeister series [Ries-Kautt a nd Ducruix, J. Biol. Chem. 264 (1989) 745]. In the present paper, we u sed small angle X-ray scattering (SAXS) to characterize the influence of various salts on the protein-protein interactions of undersaturated lysozyme solutions at constant pH (4.5) and temperature (18 degrees C ). The results show that lysozyme in a low ionic strength buffer prese nts repulsive protein-protein interactions. Addition of increasing con centrations of salts gradually leads from repulsive to attractive inte ractions demonstrating the ability of a given protein to change its in teractive behavior with additives. While cations (Li+, Na+, K+, NH4+, Cs+) all showed similar effects, large differences were observed betwe en anions in their efficiency to modify the interaction potentials. Th e order of the anions (SCN-, paratoluene sulfonate, NO3-, Cl-, H2PO4-) was found to be the same as that observed for their effectiveness in reducing lysozyme solubility and inducing crystallization.