Introduction to Membrane Proteins

  • What is membrane protein?

    Biomembrane is a barrier between cells and the outside world, which participates in and regulates various physiological activities of cells. Biomembranes are responsible for complex biochemical reactions in tissues and are essential for energy storage and cell-to-cell communication. Biomembranes are mainly composed of proteins and lipids. Lipid bilayers determine the basic structure of biomembranes. Protein is the cause of most membrane functions. Protein plays an indispensable role in various physiological functions of membrane.

    The protein contained in biomembrane is called membrane protein, which is the main undertaker of biomembrane function. Membranes from different sources often contain different membrane proteins, which match their functional specificity. Membrane proteins of different organisms vary a lot, so as the membrane proteins of different tissues from the same organism. Even if the same cell has different membranes, theircomposition is different.

    Types of membrane proteins

    According to the difficulty of protein separation and the location of protein distribution in the membrane, membrane proteins can be basically divided into three categories: extrinsic membrane proteins or peripheral membrane proteins, intrinsic membrane proteins or integral proteins and lipid anchored proteins. Membrane proteins include glycoproteins, transport proteinsand enzymes. Usually there are some sugars attached to the membrane proteins. These sugars are equivalent to transporting signals to the cells through changes in the molecular structure of the sugars themselves.

    The extrinsic membrane proteins

    The extrinsic membrane proteins are distributed on the inner and outer surface of the membrane, accounting for about 20%-30% of the membrane proteins. They are mainly water-soluble proteins on the inner surface. They bind indirectly to the membrane through ionic bonds, hydrogen bonds and polar heads of membrane lipid molecules, or through interactions with the inner proteins.

    Peripheral membrane proteins bind to hydrophilic domains of proteins and polar heads of lipids through electrostatic interaction and hydrogen bonding. Peripheral membrane proteins may act as regulators of membrane-binding enzymes, or may restrict the mobility of membrane-bound proteins by anchoring them to intracellular domains. For example, that reversibly bind to acid membrane phospholipids, which are rich in the cytoplasmic surface of lipid membranes and organelles.

    The intrinsic membrane proteins

    The intrinsic membrane proteins account for 70%-80% of the membrane proteins. They are amphiphilic mediators and can be embedded in lipid bilayer molecules to varying degrees. Some of them run through the whole lipid bilayer, and both ends are exposed to the inner and outer surface of the membrane. This type of membrane protein is also called transmembrane protein. The part of the inner membrane protein exposed outside the membrane contains more polar amino acids, which is hydrophilic and close to the hydrophilic head of the phospholipid molecule. The membrane protein embedded in the lipid bilayer consists of some non-polar amino acids, which bind closely to the hydrophobic tail of the lipid molecule.

    It is estimated that 1/4-1/3 of human genes encode intrinsic membrane proteins. Membrane intrinsic proteins are strongly bound to membranes and can only be dissolved by detergents, organic solvents or denaturants that interfere with hydrophobic interactions. The strong binding of proteins within the membrane to the membrane results from the hydrophobic interaction between the hydrophobic domains of lipids and proteins in the membrane. Some proteins contain a single hydrophobic sequence in the middle of their peptide chain or at their amino and carboxyl ends, while others contain multiple hydrophobic sequences.

    The lipid anchored proteins

    Lipid anchored proteins are anchored on the plasma membrane by inserting covalently linked lipid molecules into the lipid bilayer of the membrane.The water-soluble protein is located outside the lipid bilayer.

    Functions of membrane proteins

    Membrane proteins have many functions. Membrane proteins play an important role in many biological activities, such as cell proliferation and differentiation, energy conversion, signal transduction and material transport. It is estimated that about 60% of the drug targets are membrane proteins.

    Many membrane proteins form glycoprotein complexes by covalent binding. Most of them occur on the plasma membrane, while the endomembrane contains few covalently bound sugars. Glycoproteins play an important role in cell recognition, adsorption and signal transduction. Among them, serine, threonine and aspartic acid are the sites where protein covalently binds to sugar. Glycoprotein chains on cell surface can directly affect protein folding, stability and location on the membrane, so they play an important role in the specific binding of ligands to cell surface. Moreover, glycosyl groups on membrane proteins can form extracellular coatings, protect cells from mechanical and chemical damage, and maintain the distance between cells and external objects and other cells, and maintain cell independence.

    Membrane proteins act as "carriers" to transport substances into and out of cells. Some membrane proteins are specific receptors for hormones or other chemicals, such as thyrotropins from the pituitary on thyroid cells. There are also various enzymes on the membrane surface, which enable specific chemical reactions to take place on the membrane, such as the synthesis of phospholipids catalyzed by the endoplasmic reticulum membrane. The recognition function of cells is also determined by the proteins on the membrane surface. These proteins are often surface antigens. Surface antigens can bind to specific antibodies, such as a protein antigen HLA on the surface of human cells, which is a dimer with many changes. Different people have different HLA molecules. When transplanting organs, the transplanted organs are often excluded. This is because the HLA molecules of implanted cells are not accepted by the recipients.