The art of crystallizing membrane proteins

Abstract

In the past decade, several research groups have successfully prepared large, single crystals of integral membrane proteins for X-ray diffraction analysis. While no simple set of protocols yet exists, the general strategies for designing membrane protein crystallization experiments have become clearer. All X-ray-quality crystals of membrane proteins were grown from preparations of detergent-solubilized protein, where the heterogeneous natural lipids from the membrane have been replaced by a homogeneous detergent environment. It is the preparation of such monodisperse, isotropic solutions of membrane proteins that has allowed the successful application of the standard crystallization methods routinely used on soluble proteins. In this article, we address how the physical behavior of the detergent, in the form of micelles or protein-detergent aggregates, affects crystallization. We also discuss the roles of the detergent, salt, and precipitant in the crystallization process and the adaptation of published protocols to new membrane protein systems. We focus on the polyethylene glycol/NaCl/β-octylglucoside system as a model and show that the presence of detergent imposes several types of constraints on the crystallization conditions. The general conclusion is that many integral membrane proteins could be crystallized if pure and monodisperse preparations in a suitable detergent system can be prepared.