Ratiocinative screen of eukaryotic integral membrane protein expression and solubilization for structure determination

A Flexible and Scalable High-Throughput Platform for Recombinant Membrane Protein Production

Integral membrane proteins (MP) are implicated in many disease processes and are the primary targets of numerous marketed drugs. Despite recent advances in the areas of MP solubilization, stabilization, and reconstitution, it remains a time-consuming task to identify the combination of constructs and purification conditions that will enable MP structure-function studies outside of the lipid bilayer. In this chapter, we describe a strategy for rapidly identifying and optimizing the solubilization and purification conditions for nearly any recombinant MP, based on the use of a noninvasive fluorescent probe (His-Glow) that specifically binds to the common hexahistidine affinity tag of expressed targets. This His-Glow approach permits fluorescent size-exclusion chromatography (FSEC) without the need for green fluorescent protein (GFP) fusion. A two-stage detergent screening strategy is employed at the solubilization stage, whereby appropriate detergent families are identified first, followed by optimization within these families. Screening up to 96 unique combinations of solubilization conditions and constructs can be achieved in less than 24 h. At the outset of each new project, we screen six different detergents for each construct and the subsequent implementation of a simple thermostability challenge further aids in the identification of constructs and conditions suitable for large-scale production. Our strategy streamlines the parallel optimization of appropriate production conditions for multiple MP targets to rapidly enable downstream biochemical, immunization, or structural studies.

Expression and purification of human and Saccharomyces cerevisiae equilibrative nucleoside transporters

Nucleosides play an essential role in the physiology of eukaryotes by acting as metabolic precursors in de novo nucleic acid synthesis and energy metabolism. Nucleosides also act as ligands for purinergic receptors. Equilibrative nucleoside transporters (ENTs) are polytopic integral membrane proteins that aid in regulating plasmalemmal flux of purine and pyrimidine nucleosides and nucleobases. ENTs exhibit broad substrate selectivity across different isoforms and utilize diverse mechanisms to drive substrate flux across membranes. However, the molecular mechanisms and chemical determinants of ENT-mediated substrate recognition, binding, inhibition, and transport are poorly understood. To determine how ENT-mediated transport occurs at the molecular level, greater chemical insight and assays employing purified protein are essential. This article focuses on the expression and purification of human ENT1, human ENT2, and Saccharomyces cerevisiae ScENT1 using novel expression and purification strategies to isolate recombinant ENTs. ScENT1, hENT1, and hENT2 were expressed in W303 Saccharomyces cerevisiae cells and detergent solubilized from the membrane. After detergent extraction, these ENTs were further purified using immobilized metal affinity chromatography and size exclusion chromatography. This effort resulted in obtaining quantities of purified protein sufficient for future biophysical analysis.

FUN26 (function unknown now 26) protein from saccharomyces cerevisiae is a broad selectivity, high affinity, nucleoside and nucleobase transporter

Equilibrative nucleoside transporters (ENTs) are polytopic integral membrane proteins that transport nucleosides and, to a lesser extent, nucleobases across cell membranes. ENTs modulate efficacy for a range of human therapeutics and function in a diffusion-controlled bidirectional manner. A detailed understanding of ENT function at the molecular level has remained elusive. FUN26 (function unknown now 26) is a putative ENT homolog from S. cerevisiae that is expressed in vacuole membranes. In the present system, proteoliposome studies of purified FUN26 demonstrate robust nucleoside and nucleobase uptake into the luminal volume for a broad range of substrates. This transport activity is sensitive to nucleoside modifications in the C(2')- and C(5')-positions on the ribose sugar and is not stimulated by a membrane pH differential. [(3)H]Adenine nucleobase transport efficiency is increased ∼4-fold relative to nucleosides tested with no observed [(3)H]adenosine or [(3)H]UTP transport. FUN26 mutational studies identified residues that disrupt (G463A or G216A) or modulate (F249I or L390A) transporter function. These results demonstrate that FUN26 has a unique substrate transport profile relative to known ENT family members and that a purified ENT can be reconstituted in proteoliposomes for functional characterization in a defined system.

An efficient strategy for high throughput screening of recombinant integral membrane protein expression and stability

Membrane proteins account for about 30% of the genomes sequenced to date and play important roles in a variety of cellular functions. However, determining the three-dimensional structures of membrane proteins continues to pose a major challenge for structural biologists due to difficulties in recombinant expression and purification. We describe here a high throughput pipeline for Escherichia coli based membrane protein expression and purification. A ligation-independent cloning (LIC)-based vector encoding a C-terminal green fluorescence protein (GFP) tag was used for cloning in a high throughput mode. The GFP tag facilitated expression screening in E. coli through both cell culture fluorescence measurements and in-gel fluorescence imaging. Positive candidates from the GFP screening were subsequently sub-cloned into a LIC-based, GFP free vector for further expression and purification. The expressed, C-terminal His-tagged membrane proteins were purified via membrane enrichment and Ni-affinity chromatography. Thermofluor technique was applied to screen optimal buffers and detergents for the purified membrane proteins. This pipeline has been successfully tested for membrane proteins from E. coli and can be potentially expanded to other prokaryotes.

Unlocking the eukaryotic membrane protein structural proteome

Most of the 231 unique membrane protein structures (as of 3/2010) are of bacterial membrane proteins (MPs) expressed in bacteria, or eukaryotic MPs from natural sources. However eukaryotic membrane proteins, especially those with more than three membrane crossings rarely succumb to any suitable expression in bacterial cells. They typically require expression in eukaryotic cells that can provide appropriate endoplasmic reticulum, chaperones, targeting and post-translational processing. In evidence, only approximately 20 eukaryotic MP structures have resulted from heterologous expression. This is required for a general approach to target particular human or pathogen membrane proteins of importance to human health. The first of these appeared in 2005. Our review addresses the special issues that pertain to the expression of eukaryotic and human membrane proteins, and recent advances in the tool kit for crystallization and structure determination.

Overexpression and purification of integral membrane proteins in yeast

The budding yeast Saccharomyces cerevisiae is a viable system for the overexpression and functional analysis of eukaryotic integral membrane proteins (IMPs). In this chapter we describe a general protocol for the initial cloning, transformation, overexpression, and subsequent purification of a putative IMP and discuss critical optimization steps and approaches. Since expression and purification are often the two predominant hurdles one will face in studying this difficult class of biological macromolecules the intent is to outline the general workflow while providing insights based upon our collective experience. These insights should facilitate tailoring of the outlined protocol to individual IMPs and expression or purification routines.