Representing one third of the proteins encoded by an organism's genome and 60 % of all human drug targets, membrane proteins are ubiquitous and hold the key to structure-based design of therapeutics. Despite the great interest surrounding membrane proteins, they are grossly underrepresented in the Protein Data Bank due in large part to the inherent difficulties of working with macromolecules with extensive hydrophobic surfaces. Detergents are typically used to isolate and characterize membrane proteins, but maintenance of the native protein structure in detergent solutions presents a major challenge. Another bottleneck to structural determination of membrane proteins is the production of high-quality, three-dimensional crystals for x-ray diffraction, which remains a difficult and largely empirical task due to the complexity of the crystallizing solutions and the vastness of the multi-dimensional parameter space. Crystallization of membrane proteins is favored near the phase boundaries of surfactant solutions, and we find here that more specific characteristics of the surfactant and polymer phase behavior and resulting surfactant microstructure may play a major role in facilitating the interactions required for crystallization.

The objective of this work is to understand the stabilizing role of detergents, while identifying specific detergent properties that are predictive of the effectiveness towards stabilization and crystallization of membrane proteins. The stability of E. coli diacylglycerol kinase was studied in detergent mixtures in order to gain an understanding of the cause of protein instability in detergent solutions. The adapted stability assay revealed that changes in the mixed micelle composition correlate directly with the thermostability of the protein. Excess concentrations of small amphiphiles relative to the solubilizing detergent can significantly impact micelle composition and structure and adversely affect protein stability. Cryogenic-transmission electron microscopy images of protein-free solutions near several reported crystallization conditions reveal microdispersions consisting of a dense, surfactant-rich phase interspersed within the bulk solvent phase. The microstructure of the surfactant-rich phase varies from elongated micelles arranged in a hexagonal lattice to a randomly branched micellar network. The existence of such microstructures and the intermicellar ordering, which are both reminiscent of the mesophases that are used in the crystallization of membrane proteins in meso, suggest that a similar mechanism may be responsible for 3D crystallization in detergents.