Effects of PEG on detergent micelles: implications for the crystallization of integral membrane proteins

The solubilization of integral membrane proteins with detergents produces protein-detergent complexes (PDCs). Interactions between the detergent moieties of PDCs contribute significantly to their behavior. The effects of the precipitating agent polyethylene glycol (PEG) upon these detergent-detergent interactions have been examined, focusing on the detergent system used to crystallize the bacterial outer membrane protein OmpF porin. Static and dynamic light scattering were used to assess the effects of temperature and concentration upon the hydrodynamic size distribution and the aggregation state of detergent micelles and a phase diagram for micellar solutions was mapped. Estimates of the second osmotic virial coefficient obtained from static light-scattering measurements on micelles were shown to accurately reflect the thermodynamic quality of the solvent. Solvent quality decreases as the consolute boundary is approached, suggesting micelle-micelle attractive forces help to organize PDCs into crystalline aggregates near the cloud point. An apparent increase in micelle mass is observed as the solution approaches the cloud point. These results raise the possibility that the detergent-mediated aggregation of PDCs and/or slight changes in micelle geometry may prove to be important in the nucleation of membrane protein crystals.