Biological membranes are barriers to cells or organelles. The basic unit of most biological membranes is phospholipids, but they also contain a large number of proteins. These proteins, which are interspersed among or loosely attached to phospholipids, are membrane proteins. In eukaryotes and prokaryotes, membrane proteins represent about 20 to 30% of the proteome. Membrane proteins are not only an important component of biological membranes, but also the main undertaker of biological membranes function. Membrane proteins have various functions, including nutrient uptake, energy generation, cellular signaling, ion transport, drug resistance, and maintenance of the cellular structure. Furthermore, mutations or improper folding of membrane proteins is associated with many known diseases. Therefore, they are a very important drug target.
Classification of Membrane Proteins
Membrane proteins can be classified as either peripheral (extrinsic) or integral (intrinsic) .
Fig. 1. The peripheral and integral membrane proteins.
- Peripheral proteins are held to the membrane surface primarily through electrostatic or hydrogen bonds, which are essentially water-soluble globular proteins. Therefore, the peripheral membrane proteins can be readily removed from biological membranes without disrupting membrane by simply changing the ionic strength of the medium or adjusting the pH value. The removed protein is freely soluble in water. Peripheral proteins can also be divided into two basic types: those that are attached on the surface of integral proteins and those that are attached on the phospholipid anionic head group .
Fig. 2. Peripheral protein attached to an integral protein (left) or to anionic phospholipids (right).
- Integral membrane proteins are embedded in the membrane hydrophobic interior to varying extents. They interact extensively with membrane lipids, so they can only be removed from the membrane by more stringent measures. Integral membrane proteins are usually extracted using detergent or mechanical rupture of membrane. The extracted protein is water-insoluble aggregates. Integral membrane proteins are the target of most studies, which accounts for 70 ~ 80% of membrane protein species.
Structure of Membrane Proteins
At present, most studies on the structure of membrane proteins have focused on integral membrane proteins. Integral membrane proteins are classified by their overall structure into two dominant classes: α-helical bundles and β-barrels .
Fig. 3. The two structure of integral membrane proteins: α-helical bundles and β-barrels.
- α-Helical bundles: α-helix is an important secondary structural unit, which is one of the bases of protein tertiary structure and plays an important role in protein structure and function. In the majority of integral membrane proteins each transmembrane segment (TMS) is an α-helix of twenty or more predominantly nonpolar residues and the presence of one or two polar residues is tolerated. α-helical membrane proteins often appear in plasma membrane, endoplasmic reticulum and mitochondrial intima. The first membrane protein observed by electron microscopy to consist of an α-helical core was bacteriorhodopsin (as shown in Fig. 4) .
Fig. 4. The fold of α-helical structure of bacteriorhodopsin (an integral membrane proteins).
- β-barrels: β-barrels typically consist of an even number (8 to 26) of amphiphilic β-strands crossing the bilayer at a tilt of ∼45°, each containing 9 to 11 residues hydrogen-bonded to the adjacent strands. Membrane-spanning β-barrels are found in the outer membranes of Gram negative bacteria as well as mitochondria and chloroplasts, where they function in transport, phage binding, catalysis, and adhesion.
Functions of Membrane Proteins
Membrane proteins perform a variety of vital functions to the survival of organisms:
- Transport: Transport is carried out by membrane transporters. Membrane transporters are divided into carrier proteins and channel proteins. Carrier proteins can bind to specific solute molecules and mediate solute molecules' transmembrane movement through their own conformational changes. Channel proteins allow appropriately sized molecules and charged molecules to pass through in a simple free diffusion motion.
- Recognition: Recognition is performed by receptor proteins. Membrane receptor proteins relay signals between the cell's internal and external environments.
- Connection: Through some specific membrane proteins, the adjacent cell membranes are linked to each other, playing synergistic effect.
- Catalysis: Membrane enzymes may have many activities, such as oxidoreductase, transferase or hydrolase, which can catalyze a variety of reactions that the survival of organisms.
- Protection: Most of carbohydrates on cell membranes will form glycoproteins. Because of high viscosity, glycoproteins act as a lubricant on the cell surface, preventing proteolytic hydrolysis. At the same time, they can also prevent the invasion of bacteria and viruses.
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- Hedin L. E., et al. An introduction to membrane proteins[J]. Journal of proteome research, 2011, 10(8): 3324-3331.
- Stillwell W. Chapter 6-Membrane Proteins[J]. An Introduction to Biological Membranes (Second Edition), 2016.
- Mary Luckey, Chapter 1: Introduction to the Structural Biology of Membrane Proteins [J]. Computational Biophysics of Membrane Proteins, 2016, pp. 1-18.
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