alpha-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 tr ansparency. They are polydisperse oligomers, composed by more than 40 subun its and have a molecular weight between 800 and 900 kDa. We have studied th e effect on the protein-protein interactions as a function of different par ameters, 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 stu dy the interaction potentials between proteins in solution. The combination of SAXS and numerical simulations allows us to determine the potential par ameters from the comparison of the experimental structure factors and the c alculated structure factors. At physiological PH, the alpha -crystallin int eraction forces are repulsive (hard core and coulombic repulsive forces). T he addition of salt screens the protein charges, thus decreasing the coulom bic 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, relat ed to a depletion mechanism. (C) 2001 Elsevier Science B.V. All rights rese rved.