Identifying key factors in two-dimensional crystal production and sample preparation for structure-function studies of membrane proteins by cryo-EM

Abstract

Electron crystallography of two-dimensional crystals is a structure-determination method well suited to the study of membrane protein structure-function. Two-dimensional crystals consist of ordered arrays of protein within reconstituted lipid bilayers, an arrangement that mimics the natural membrane environment. In this work we describe our recent progress in the use of this method with three different proteins, each providing a window into a separate paradigm in the electron crystallographic pipeline. Specific crystallization conditions for human leukotriene C₄ synthase (LTC₄S) have previously been determined, but our continued refinement of purification and crystallization has identified a number of additional parameters that greatly affect crystal size and quality, and we have developed a protocol to rapidly and reproducibly grow large, non-mosaic crystals of LTC₄S. The human gamma-glutamyl carboxylase (GGCX) has also been crystallized, but is sensitive to cryo-EM sample preparation conditions and we present here the successful reproduction of crystallization and refinement of cryo-EM sample preparation conditions. Lastly, we describe our crystallization screens with the Vibrio cholerae sodium-pumping NADH:ubiquinone reductase complex (Na⁺-NQR), and identify the factors critical to membrane reconstitution of the complex, a necessary first step towards crystallization. We also describe a semi-quantitative crystal screening protocol we have developed that provides quick and accurate method to assess two- dimensional crystallization trials, and discuss some general observations in optimization of membrane protein purification and two-dimensional crystallization for electron crystallography.