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Transmembrane proteins are located at the interface between cells and the outside world, mediating the signal transduction between cells and the outside world, and performing many important cellular biological functions. For example, it is a receptor for various signaling molecules, hormones and other substrates; it is involved in the exchange of substances, energy and signal between the inside and outside of the cell membrane; it constitutes a channel for various ion transmembranes, which inputs nutrients and some inorganic electrolytes into cells, and discharges toxic or useless metabolites into cells.

Cell-free expression systems play an important role in the expression of transmembrane proteins due to their unique advantages.

In a cell-free expression system, we can modify transmembrane proteins. For example, non-natural amino acids can be added to the protein expression to realize the complex modification process that is difficult to be solved after conventional recombinant expression.

Compared with the traditional intracellular protein expression system, the cell-free system has significant advantages. For details, click the link below: https://www.cusabio.com/protein_service/In_vitro_E.coli_Expression_System.html

The types and functions of transmembrane proteins expressed by cell-free expression system are diverse. Currently, the most popular products are listed as follows:

• G-protein-coupled receptors (GPCR): G protein-coupled receptor (GPCR) is a general term for a large class of membrane protein receptors. It binds to extracellular molecules and activates internal signal transduction pathways, ultimately activating cellular responses. GPCRs mediate signals involving visual control, kidney function, tumorigenesis, immune response, and inflammation.
  For more details, please click the following link: https://www.cusabio.com/c-20796.html
• Aquaporin (AQP): Aquaporin is a protein located on the cell membrane (internal membrane protein) that forms "channels" in the cell membrane that control the flow of water in and out of cells.
  Now we have already successfully developed some active aquaporins through the cell-free expression system. Detergent plays an important role in the extraction, purification and operation of membrane proteins.
  To learn more about the use of detergent in aquaporin, this article may help you: The Nature of Detergent and Its Application in Membrane Proteins
• Ion channel: Ion channel is a passive transport pathway for various inorganic ions across the membrane and an important way for living cells to carry out metabolic activities and material exchange. Ion channels have great significance for the realization of various functions of cells.
  You may want to learn more about the function of ion channels, please click here: A Switch that Controls the Entry and Exit of Ions
• ATP-Binding Cassette (ABC): ATP-binding cassette (ABC) protein can bind and hydrolyze ATP and use energy to drive various molecules across the plasma membrane as well as the endoplasmic reticulum (ER), peroxisome and mitochondrial inner membrane. It is involved in the transport of specific molecules on the lipid membrane and the resistance in all organisms.
  More information about ABC structure, function, and its relationship with disease can be found in this article: A transport machine -- ATP-binding cassette
• Human leucocyte antigen (HLA): HLA is a protein or antigen found on the surface of cells in the body, which specifically presents short peptides to T cells and haves a key role in the immune defense of the body. The clinical application of HLA is mainly the matching of donor and recipient in organ transplantation.
  If you are interested in information about it, this article fits your needs: Human Leukocyte Antigen: Distinguish between Friend and Foe

Transmembrane protein products expressed by cell-free expression systems can be used to study the functions of transmembrane proteins. The integration of transmembrane proteins into vesicles for structural and functional studies is a hot topic in membrane protein research.

According to its structure, transmembrane proteins can be classified into alpha helix and β - barrel membrane proteins. For details, please refer to the article: A Resume for Transmembrane Proteins.