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.

Static light scattering studies of OmpF porin: implications for integral membrane protein crystallization.

Integral membrane proteins carry out some of the most important functions of living cells, yet relatively few details are known about their structures. This is due, in large part, to the difficulties associated with preparing membrane protein crystals suitable for X-ray diffraction analysis. Mechanistic studies of membrane protein crystallization may provide insights that will aid in determining future membrane protein structures. Accordingly, the solution behavior of the bacterial outer membrane protein OmpF porin was studied by static light scattering under conditions favorable for crystal growth. The second osmotic virial coefficient (B22) was found to be a predictor of the crystallization behavior of porin, as has previously been found for soluble proteins. Both tetragonal and trigonal porin crystals were found to form only within a narrow window of B22 values located at approximately -0.5 to -2 X 10(-4) mol mL g(-2), which is similar to the "crystallization slot" observed for soluble proteins. The B22 behavior of protein-free detergent micelles proved very similar to that of porin-detergent complexes, suggesting that the detergent's contribution dominates the behavior of protein-detergent complexes under crystallizing conditions. This observation implies that, for any given detergent, it may be possible to construct membrane protein crystallization screens of general utility by manipulating the solution properties so as to drive detergent B22 values into the crystallization slot. Such screens would limit the screening effort to the detergent systems most likely to yield crystals, thereby minimizing protein requirements and improving productivity.