Existing and emergent roles for surfactants in the three-dimensional crystallization of integral membrane proteins

The determination of protein structures by X-ray crystallography necessitates the formation of well-ordered three-dimensional crystals that are grown from protein solutions. For integral membrane proteins, the protein solute consists of a protein surrounded by surfactant molecules; therefore, utilization of surfactant properties to improve the probability of membrane protein crystallization may provide the basis of rational crystallization methods. Properties of surfactants that may be germane to membrane protein crystallization include consolute boundaries, the second virial coefficient B22, and critical phenomena. Also, the non-lamellar lipidic cubic phase (LCP) has been shown to be a unique matrix in which to form membrane protein crystals. These physical and physical–chemical descriptors of surfactants (or lipids) are discussed in the context of relevance or potential relevance to three-dimensional membrane protein crystallization. Other topics covered include the development of crystallization screens based upon surfactant properties, novel surfactants, and the use of additives to improve crystal quality.