Transient Expression of Recombinant Membrane-eGFP Fusion Proteins in HEK293 Cells
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HEK 293 cells
Protein purification
Ectopic expression
HEK 293 cells
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Understanding the mechanisms of membrane protein function is critical for biomedical research and drug discovery as membrane proteins constitute ∼30% of the proteins encoded by the genomes of both lower and higher organisms and are targets for two-thirds of approved drugs worldwide. Significant progress has been made in engineering host expression systems for large-scale production of membrane proteins and in determining their three-dimensional high-resolution structures. Despite these efforts, the study of membrane proteins at the atomic level is challenging due to poor expression and extraction, low yields of functional protein, and the complexity and heterogeneity of source membranes. Structural and spectroscopic studies of any membrane protein require that the protein be extracted from its native membranes into a membrane-mimetic stable environment, which is often achieved by the use of detergents. Unfortunately, there is no magic detergent that can extract all membrane proteins and successful extraction often requires a thorough screen of detergents. Furthermore, membrane protein purification in general and the detergents used are very expensive, which puts a financial constraint on sophisticated membrane protein studies. To overcome this hurdle, a dual-detergent strategy has recently been developed and successfully applied to purify various classes of pure, stable, and functionally relevant membrane proteins in a cost-effective manner. This strategy uses an inexpensive detergent for solubilization of the desired protein from membranes and a second detergent during protein purification. In the Basic Protocol, we describe the dual-detergent strategy to significantly reduce the overall purification cost of a bacterial membrane protein using the magnesium ion channel MgtE as an example. Support Protocols are also provided for selecting a suitable E. coli strain for protein expression and the optimal detergent(s) for membrane protein solubilization. © 2022 Wiley Periodicals LLC. Basic Protocol: Expression, membrane solubilization, and cost-effective purification of MgtE Support Protocol 1: Selecting a suitable E. coli strain for optimal protein expression Support Protocol 2: Identification of suitable detergents for membrane protein solubilization.
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Protein purification
Free-flow electrophoresis
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Protein purification
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Protein purification
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Proteome
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Phase separation is a simple, efficient, and cheap method to purify and concentrate detergent-solubilized membrane proteins. In spite of this, phase separation is not widely used or even known among membrane protein scientists, and ready-to-use protocols are available for only relatively few detergent/membrane protein combinations. Here, we summarize the physical and chemical parameters that influence the phase separation behavior of detergents commonly used for membrane protein studies. Examples for the successful purification of membrane proteins using this method with different classes of detergents are provided. As the choice of the detergent is critical in many downstream applications (e.g., membrane protein crystallization or functional assays), we discuss how new phase separation protocols can be developed for a given detergent buffer system.
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The human proteome project will demand faster, easier, and more reliable methods to isolate and purify protein targets. Membrane proteins are the most valuable group of proteins since they are the target for 70–80% of all drugs. Perbio Science has developed a protocol for the quick, easy, and reproducible isolation of integral membrane proteins from eukaryotic cells. This procedure utilizes a proprietary formulation to facilitate cell membrane disruption in a mild, nondenaturing environment and efficiently solubilizes membrane proteins. The technique utilizes a two‐phase partitioning system that enables the class separation of hydrophobic and hydrophilic proteins. A variety of protein markers were used to investigate the partitioning efficiency of the membrane protein extraction reagents (Mem‐PER) (Mem‐PER is a registered trademark of Pierce Biotechnology, Inc) system. These included membrane proteins with one or more transmembrane spanning domains as well as peripheral and cytosolic proteins. Based on densitometry analyses of our Western blots, we obtained excellent solubilization of membrane proteins with less than 10% contamination of the hydrophobic fraction with hydrophilic proteins. Compared to other methodologies for membrane protein solubilization that use time‐consuming protocols or expensive and cumbersome instrumentation, the Mem‐PER reagents system for eukaryotic membrane protein extraction offers an easy, efficient, and reproducible method to isolate membrane proteins from mammalian and yeast cells.
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GFP (Green Fluorescent Protein) is well known for its unique chromophore which is formed by autocatalytic cyclization of a polypeptide backbone of Ser65, Tyr66 and Gly67, and is able to emit green visible light. Due to unusual chromophore responsible for the fluorescence GFP and its mutants (e.g., EGFP) have become widely used reporter proteins in molecular biology and biotechnology. GFP can easily be fused to any protein of interest and co-expressed in cells; the GFP fluorescence is then used to visualize the distribution, transport and aggregation of the protein in the cell. However, GFP has a tendency to aggregate itself, and also formation of its chromophore critically depends on the presence of reducing agents. Therefore we have undertaken spectroscopic kinetic studies of EGFP folding and aggregation as a function of pH, and in the presence of various reducing agents, to study the competition between these two processes. The best conditions for folding of EGFP provides BME as a reducing agent. Aggregation of EGFP depends strongly on pH, and on the concentration of the protein. The careful control experiments must therefore be performed during investigations of proteins fused with EGFP, especially at pH lower than 7.
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