α-crystallin interaction forces studied by small angle X-ray scattering and numerical simulations

alpha -crystallins are lens specific proteins, which play a role in lens transparency. They are polydisperse oligomers, composed by more than 40 subunits and have a molecular weight between 800 and 900 kDa. We have studied the effect on the protein-protein interactions as a function of different parameters, such as temperature, addition of salt and addition of polymers (in the case of bovine alpha -crystallins, a variation of PH induces a loss of subunits). Small angle X-ray scattering (SAXS) is a convenient tool to study the interaction potentials between proteins in solution. The combination of SAXS and numerical simulations allows us to determine the potential parameters from the comparison of the experimental structure factors and the calculated structure factors. At physiological PH, the alpha -crystallin interaction forces are repulsive (hard core and coulombic repulsive forces). The addition of salt screens the protein charges, thus decreasing the coulombic repulsive forces, but is not sufficient to induce attractive forces. In order to reach an attractive regime, the addition of polymer is necessary. As previously shown with ATCase, the addition of polyethylene glycol, e.g. 4% PEG 8000, is sufficient to induce a short-range attractive force, related to a depletion mechanism. (C) 2001 Elsevier Science B.V. All rights reserved.