What is the purpose of transmembrane proteins?

Understanding:

•  Membrane proteins are diverse in terms of structure, position in the membrane and function

    
Phospholipid bilayers are embedded with proteins, which may be either permanently or temporarily attached to the membrane

• Integral proteins are permanently attached to the membrane and are typically transmembrane (they span across the bilayer)
• Peripheral proteins are temporarily attached by non-covalent interactions and associate with one surface of the membrane  

  Transmembrane proteins are proteins which are situated in the lipid membrane of cells. They have transmembrane spanning regions which pass through the lipid bilayer of the cell membranes any number of times depending on the protein in question. There are multiple families of transmembrane proteins and each protein has a specific role. The two main forms of transmembrane proteins are channels and carriers. Channels form a constant pore across the plasma membrane and allow for fast diffusion of molecules. Carriers (or transporters/pumps) bind to the solute that is going to cross the membrane and undergo a conformational change so that the binding site closes on the side of the membrane the molecule bound to and opens on the opposite side releasing the substance[1]. Carrier proteins can be divided into three different groups:

  Uniporters transport only one molecule across the plasma membrane at a time, unlike symporters and antiporters which transport two different molecules at once (cotransport). However, symporters transport two molecules in the same direction across the membrane and antiporters transport the molecules in opposite directions[2].

  A common antiporter example is the sodium-potassium ATPase pump. 3 sodium ions start by attaching themselves to the binding site of the carrier protein. ATPase breaks down adenosine triphosphate which results in an inorganic phosphate molecule binding to the transmembrane protein. A conformational change in the protein is triggered in which the 3 sodium ions are released outside of the membrane. The 2 potassium ions are now enabled to bind to the same protein which stimulates the dephosphorylation of the attached phosphate. Conformational change again occurs causing the protein to return to its original shape. By doing this, the 2 potassium ions are released in the membrane's interior. This is very important in sustaining life[3].

  These proteins are normal highly structured as the amino acid primary protein sequence allows for the formation of alpha-helices or beta-sheets.

  For extraction, transmembrane protein require detergent or any non-polar solvent. Some of them (beta-sheets) however can be extracted using a denaturing agent.

  G Protein Linked Receptors:

  This is a form of transmembrane protein consisting of 7 membrane domains and 1 ligand binding domain, for example, the Glycolipids, the most complex form of these been the gangliosides. Associated to these domains via a binding domain is a trimeric G protein, consisting of 3 heterologous subunits (α, β, γ). The α-subunit is the catalytically active region, more specifically GTPase[4].

  The binding of a signal ligand induces a conformation change within the α-subunit causing GDP to be released, consequently binding GTP. It then dissociates from the connected β and γ units and is now said to be activated.

  Membrane proteins are proteins that are part of or interact with cell membranes, and they are responsible for carrying out the majority of the functions of these membranes. Membrane proteins account for approximately one-third of human proteins and are responsible for regulating processes that help biological cells survive.

  
  Image Credit: sciencepics/Shutterstock.com

  Membrane proteins have a range of different structures and are also situated in different areas of the membrane. They carry out a diverse range of functions, and the number of proteins and the types of proteins present on a particular membrane can vary.

  Membrane protein structure

  Cell membranes are made up of two phospholipid bilayers, which are called leaflets. These leaflets are present on all cells, forming a barrier that surrounds each cell. Membrane proteins are found on these phospholipid bilayers or they interact with these phospholipid bilayers.

  There are non-polar membrane proteins that are hydrophobic (water repellent) and polar membrane proteins that are hydrophilic (able to mix with water), that are found inside the lipid bilayer. They are directly involved with the lipid bilayers that make a barrier around every cell.

  Integral membrane proteins are a permanent fixture on the membrane.

  Peripheral membrane proteins are not a permanent part of a membrane and can have hydrophobic, electrostatic, and other non-covalent interactions with the membrane or the integral proteins.

  Integral proteins come in different types, such as monotopic, bitopic, polytopic, lipid-anchored proteins, or transmembrane proteins.

  Monotopic integral proteins are only attached to one of the cell’s two leaflets.

  Bitopic integral proteins are transmembrane proteins that can span lipid bilayers once. Polytopic proteins are also transmembrane proteins, which span lipid bilayers more than once.

  A lipid-anchored protein has a covalent attachment to lipids that are embedded in the phospholipid bilayer.

  Membrane protein functions

  There is a diverse range of functions that membrane proteins carry out. These include:

  • Junctions: connecting two cells together

  Enzymatic functions

  All enzymes are a type of protein. As a result, a membrane protein that is embedded into the membrane can sometimes be an enzyme, which may have its active site facing substances outside of the lipid bilayer.

  These types of enzymatic membrane proteins can work in teams to carry out the steps in a particular metabolic pathway, for instance breaking down lactose into carbohydrates and then monosaccharides.

  Transportation

  Membrane proteins can allow hydrophilic molecules to pass through the cell membrane. Transport membrane proteins come in many forms, and some require energy to change shape and actively move molecules and other substances across the cell membrane. They do this by releasing ATP to use as an energy source.

  • Anchorage: become points of attachment for the cytoskeleton and the extracellular matrix

  Signal transduction

  Some membrane proteins can feature a binding site. These binding sites are characterized by specific shapes that match the shape of a chemical messenger. For example, these chemical messengers can be hormones.

  When a hormone meets with the cell wall, it will connect with a receptor membrane protein that is embedded inside the cell wall. The hormone can change the receptor protein and cause a specific reaction, depending on the type of hormone or other substance, will take place within the cell.

  Cell recognition

  Another important function of membrane proteins is in identification and recognition between cells. This particular function is useful in the immune system, as it helps the body to recognize foreign cells that may be causing infection, for instance. Glycoproteins are one type of membrane protein that can carry out cell recognition.

  Intercellular joining

  Adjacent cells may have membrane proteins that connect in a range of different junctions. Gap junctions and tight junctions.

  This function helps cells to communicate with one another, and to transfer materials between one another.

  Attachment

  Membrane proteins are important in the cytoskeleton, the system of filaments and fibers in the cytoplasm of a cell, and the extracellular matrix (ECM), which is the network of macromolecules found outside of cells, such as collagen, enzymes, and glycoproteins, to membrane proteins.

  Attaching filaments or fibers in the cytoplasm found throughout the cell can help the cell to maintain its particular shape. It also keeps the location of membrane proteins stable.

  Attaching membrane proteins to the extracellular matrix can help the ECM to mediate changes that occur in extracellular and intracellular environments.

  Membrane Proteins in Disease

  Several diseases are linked to mutations within membrane proteins. One example is a mutation called V509A, found in the thyrotropin receptor, thyrotropin being a hormone secreted by the pituitary gland that regulates the production of thyroid hormones.

  This mutation increases the activity of the thyrotropin receptor and leads to congenital hyperthyroidism, a condition that can cause changes in mood, sleep problems, and stomach problems.

  Other diseases that are linked to mutations in membrane proteins include hereditary deafness, Charcot-Marie-Tooth disease, which damages the peripheral nerves outside the central nervous system, and Dejerine-Sottas syndrome, which affects a person’s ability to move.

  

  Image Credit: Explode/Shutterstock.com

  Summary

  Membrane proteins serve a range of important functions that helps cells to communicate, maintain their shape, carry out changes triggered by chemical messengers, and transport and share material.

  Membrane proteins can also play a part in disease progression, as the immune system can use membrane proteins to identify potentially harmful foreign molecules within the body.

  What is the main function of transmembrane protein?

  Only transmembrane proteins can function on both sides of the bilayer or transport molecules across it. Cell-surface receptors are transmembrane proteins that bind signal molecules in the extracellular space and generate different intracellular signals on the opposite side of the plasma membrane.

  Why are transmembrane transport proteins needed?

  Membrane transport proteins fulfill an essential function in every living cell by catalyzing the translocation of solutes, including ions, nutrients, neurotransmitters, and numerous drugs, across biological membranes.

  What are the six functions of transmembrane proteins?

  Transport. An exchange of molecules (and their kinetic energy and momentum) across the boundary between adjacent layers of a fluid or across cell membranes..

  Enzymatic Activity. ... .

  Signal Transduction. ... .

  Cell-cell Recognition. ... .

  Intercellular Joining. ... .

  Attachment to Cytoskeleton and Extracellular Matrix (ECM).

  What is the most important function of membrane proteins?

  Membrane proteins mediate processes that are fundamental for the flourishing of biological cells. Membrane-embedded transporters move ions and larger solutes across membranes, receptors mediate communication between the cell and its environment and membrane-embedded enzymes catalyze chemical reactions.