Lecture 4. Plasma Membrane (1) PDF

Structure and Function of Plasma Membrane Content 1. Plasma Membrane Basics 2. Timeline of Models to Study the Structure and Function of Plasma Membrane 3. Structure of Plasma Membrane 4. Lipids Present in Plasma Membrane 5. Function of Lipids in Plasma Membrane 6. Plasma Membrane Pr...

Structure and Function of Plasma Membrane Content 1. Plasma Membrane Basics 2. Timeline of Models to Study the Structure and Function of Plasma Membrane 3. Structure of Plasma Membrane 4. Lipids Present in Plasma Membrane 5. Function of Lipids in Plasma Membrane 6. Plasma Membrane Proteins 7. Biological Importance of Plasma Membrane Proteins 8. Fluid Mosaic Model describing Structural Aspects of Plasma Membrane 9. Plasma Membrane Fluidity 10. Biotechniques used to Study the Structural Features of Plasma Memebrane  The plasma membrane is also termed as a Cell Membrane Plasma Membrane Basics or Cytoplasmic Membrane.  The plasma membrane is present both in plant and animal cells (dynamic fluid structure that forms external boundary of cells)  It functions as the selectively permeable membrane, by permitting the entry of selective materials in and out of the cell according to the requirement.  In an animal cell, the cell membrane functions by providing shape and protects the inner contents of the cell.  Based on the structure of the plasma membrane, it is regarded as the fluid mosaic model. According to the fluid mosaic model, the plasma membranes are subcellular structures (quasi-fluid), made of a lipid bilayer in which the protein molecules are embedded. 14-09-2024 Cell Morphology: External Cellular Components 2 Timeline of Models to Study the Structure and Function of Plasma Membrane Name of Model Model Diagram Scientist Year Lipid Nature Thin layer of Lipids Overton 1890 Lipid Monolayer Langmuir 1905 Lipid layer with Non polar polar and non polar ends Polar Lipid Bilayer E. Gorter & F. Grendel 1926 Timeline of Models to Study the Structure and Function Name of the Model Model Diagram Scientist Year Lipid Bilayer plus Protein Sheets Davson and Danielli Lipid 1935 bilayer is coated with thin sheets of proteins Unit Membrane Robertson 1960 All cellular membrane share a of Plasma Membrane common underlying structure - Unit Timeline of Models to Study the Structure and Function of Plasma Membrane Name of Model Model Diagram Scientist Year Fluid Mosaic Model Singer and 1972 Nicholson  Structure of Cell Membrane  The fluid mosaic model was proposed by S.J. Components Location Singer and Garth L. Nicolson. This model explains the structure of the plasma membrane Phospholipid The main fabric of plasma membrane of animal cells as a mosaic of components such Cholesterol Between phospholipids and phospholipid as phospholipids, proteins, cholesterol, and bilayers carbohydrates. Integral proteins Embedded within phospholipid layers  These components give a fluid character to the Peripheral proteins Inner or outer surface of the phospholipid membranes. bilayer Carbohydrates Attached to proteins/lipids on outside  Each phospholipid has a hydrophilic head [5-10%] membrane layers (Forms cell pointing outside and a hydrophobic tail coat/glycocalyx) [Protects cell from [Amphipathic molecule] forming the inside of mechanical/chemical damage, specific the bilayer. transient, cell-cell adhesion]  Cholesterol and proteins are embedded in the Irrespective of the cell type, all Plasma membrane have similar components but their composition might vary. bilayer that gives the membrane a mosaic look.  Human RBCs contain 43% lipid and 49% proteins. Each component has a specific function to  Mouse liver cells contains 54% lipids and 46% proteins. perform. 14-09-2024 Cell Morphology: External Cellular Components 6 The cell membrane is primarily made up of : 1. Phospholipids 2. Proteins [Fatty acid tails]  Polar head group are in contact with intra/extra cellular aqueous phase  Non polar tails face each other (hydrophobic interior of membrane) 14-09-2024 Cell Morphology: External Cellular Components 7  Lipids of Plasma Membrane  Plasma membrane contains 3 classes of lipids 1. Phospholipids (Examples: Phosphophatidyl choline; Phosphophatidyl serine; Phosphophatidyl ethanolamine; Sphingomyelin)- Cell growth and division 2. Glycolipids (Cerebroside; Gangliosides)- Cell recognition and adhesion 3. Sterols (Cholesterol; Stigmasterol)- Cell growth and viability, and regulate the fluidity, permeability  Certain lipids (Cholesterol and Sphingo-phospholipids) are organized as aggregates in plasma membrane in the form of lipid rafts. These lipid rafts are membrane microdomains enriched with cholesterol and glycosphingolipids along with proteins.  Function of Lipid Rafts : a. Signal transduction; b. Cholesterol trafficking; c. Endocytosis  Types of Lipid Rafts: Caveolar [Flask shaped invagination having caveolin protein found in brain, micro-vessels of the nervous system, endothelial cells, astrocytes, oligodendrocytes, Schwann cells, dorsal root ganglia and hippocampal neurons] and Non-caveolar [Planar lipid rafts found in continuation with plasma membrane and contains flotillin proteins and are found in neurons]  Lipid Aggregates to interact with water occur in three forms a). Fatty acid side chain forms small spherical micellar structure (< 20 nm) b). Two lipid monolayer forms sheets c). Liposome: Closed (sealing solvent) filled vesicle 14-09-2024 Cell Morphology: External Cellular Components 8  Figure: Non-caveolar and caveolar lipid rafts (Martinez-Outschoorn et al., 2015) Figure: Lipid Aggregates  Function of Lipids in Plasma Membrane  Lipids are used for energy storage. These function primarily as anhydrous reservoirs for the efficient storage of caloric reserves.  The matrix of cellular membranes is formed by polar lipids, which consist of a hydrophobic and a hydrophilic portion. The inclination of the hydrophobic moieties to self-associate , and the tendency of the hydrophilic moieties to interact with aqueous environments and with each other, is the physical basis of the formation and stability of membranes.  This fundamental principle of amphipathic lipids is a chemical property that enabled the first cells to segregate their internal constituents from the external environment. This same principle is recapitulated within the cell to produce discrete organelles. This compartmentalization enables segregation of specific chemical reactions for the purposes of increased biochemical efficiency and restricted dissemination of reaction products.  In addition to the barrier function, lipids provide membranes with the potential for budding, fission and fusion, characteristics that are essential for cell division, biological reproduction and intracellular membrane trafficking.  Lipids also allow particular proteins in membranes to aggregate, and others to disperse. 14-09-2024 Cell Morphology: External Cellular Components 10  Plasma Membrane Proteins  Membrane Proteins are responsible for most of the dynamic processes in Plasma Membrane.  Most membrane proteins are classified as a). Peripheral [Extrinsic] or b). Integral [Intrinsic] Membrane proteins are among the most important proteins biologically because they allow the cells to communicate with their environments, they determine whether the immune system recognizes the cell as foreign or not, they are the targets of most, and perhaps all, pharmaceuticals, they control cell adhesion to form tissues, and they control important metabolic processes, including salt balance, energy production and transmission, and photosynthesis. In short, they are important in across medicine and agriculture. 14-09-2024 Cell Morphology: External Cellular Components 11  Plasma Membrane Proteins 1. Peripheral Proteins  These proteins are bound by electrostatic or hydrogen bonds to outer layer of membrane with no interaction with the hydrophobic core of lipid bilayer.  They are indirectly bound to the integral membrane protein (protein-protein interaction) or directly interact with the polar lipid head groups (protein-lipid interaction).  Most polar proteins are soluble in aqueous solution.  These proteins on plasma membrane are released from the membrane by mild extraction procedure like exposure to high ionic strength solution or extremely high/low pH (without disrupting the lipid bilayer)  Examples: Spectrin and ankyrin on RBCs membrane. Spectrin is a flexible rod-like protein that helps maintain the structure of the cell membrane and cell shape. It also helps with cell adhesion, cell spreading, and the cell cycle. Ankyrin is and adaptor protein that links membrane proteins to the cytoskeleton. It also helps resist shear stress and provides anchoring systems. 14-09-2024 Cell Morphology: External Cellular Components 12  Plasma Membrane Proteins 2. Transmembrane Proteins  Proteins held in lipid bilayer very tightly, which cannot be released via mild extraction process are called integral proteins or transmembrane proteins (one or more segments are embedded in phospholipid bilayer).  Transmembrane proteins are non-polar amino-acid residues with hydrophobic side chains which interacts with the fatty acyl groups of membrane phospholipids, thus anchoring the proteins to the membrane.  They are characterized to contain 21-26 hydrophobic residues coiled into an α-helical structure spanning the lipid bilayer.  They may be single pass (monotopic) or multi-pass (polytopic).  Examples:  Glycophorin is a major single pass transmembrane protein with 131 amino acid residues in the RBCs plasma membrane, carry sugar molecules and help determine blood groups.  Band 3 protein/chloride-bicarbonate exchanger, 95 kDa multi-pass membrane protein for the exchange of chloride and bicarbonate. 14-09-2024 Cell Morphology: External Cellular Components 13  Characteristic Features of Plasma Membrane Proteins Property Peripheral Proteins Integral Proteins Treatment Mild Treatment: extreme pH change, Hydrophobic bond breaking agents exposure to ionic solvents – Detergents, organic solvents Association with lipids Usually soluble, free of lipids Usually associates with lipids when when solubilized solubilized (lipid bilayer also gets disrupted) Solubility after Soluble and molecularly disperses in Usually insoluble or aggregated in dissociation from neutral aqueous buffer neutral aqueous buffer. Membrane Examples Spectrin, Ankyrin – RBC Membrane Glycophorin, Band 3 protein 14-09-2024 Cell Morphology: External Cellular Components 14  Classification of Membrane Proteins According to Function Membrane proteins are classified as 1. Transport Proteins i.e. Carrier or Channel Proteins 2. Catalytic Proteins i.e. F0–F1 ATP synthase in IM of mitochondria 3. Structural Proteins  Carrier Proteins are classified as i). uniporters or ii). Co-transporters (anti-porters/symporters). The transport can be either active/passive. Example: Glucose transporter/ Chloride-bicarbonate exchange protein  Channel Proteins transport solutes down the concentration gradient.  Channel Proteins work based on specific signals. The major signals which cause ion channels to open are 1. Voltage gated channels 2. Ligand gated channels Example: Aquaporins and Ionophores Channels – Form open pores through the membrane allowing the free passage of any molecule of the appropriate size. Carriers – Selectively binds and transports specific molecules such as glucose rather then forming open channels, carriers protein acts like enzymes so as to facilitates the passage of specific molecules across the membrane. 14-09-2024 Cell Morphology: External Cellular Components 15  Biological Importance of Membrane Proteins  Membrane proteins are interesting scientifically because of their key roles in controlling the processes of life.  Receptors are membrane proteins that bind to chemicals (e.g., drugs) outside of the cell, and this binding process causes a chemical response on the inside of cells. For example, morphine binds to the opioid receptor in the membranes of brain cells, and the interiors of the cells respond by reducing nerve transmission.  Ion-channels are membrane proteins that allow transport of chemical species into and out of cells. Nerve transmission, for example, is caused by a difference in ionic strengths across the membrane and this is mediated by ion channels. Ion-channels are also targets for drug discovery, and new analytical technology developed for research on receptors will benefit research on ion-channels.  Drug companies want to identify receptors (plasma membrane proteins), study what binds to them (new drug candidates), and understand what the cell and organism do in response to the binding. 14-09-2024 Cell Morphology: External Cellular Components 16  Plasma Membrane Structure 14-09-2024 Cell Morphology: External Cellular Components 17  Fluid Mosaic Model describing Structural Aspects of Plasma Membrane  In 1972, Jonathan Singer and Garth Nicolson proposed the fluid mosaic model of membrane structure, which is now generally accepted as the basic paradigm for the organization of all biological membranes.  In this model, membranes are viewed as two-dimensional fluids in which proteins are inserted into lipid bilayers. Fluid Mosaic Model Postulates that 1. Lipids and integral protein are disposed in a kind of mosaic arrangement. 2. The biological membrane are Quasi-fluid structures in which both lipids and integral proteins are able to perform translational movements within the bilayer.  The concept of fluidity implies that the main components of the membrane are held by means of non – covalent interaction and the cohesive forces between lipids and proteins are hydrophobic in character.  In fluid mosaic model the integral proteins of the membrane are intercalated into a continuous lipid layer. Integral proteins are amphipathic, with polar regions protruding from the surface and non-polar regions embedded in the hydrophobic interior of the membrane 14-09-2024 Cell Morphology: External Cellular Components 18  Plasma Membrane Fluidity  Membrane fluidity refers to the fact that lipids have considerable freedom of lateral or transverse movements. Phospholipids molecules are capable of 3 kinds of movements: 1. Rotation along its long axis; 2. Lateral diffusion by exchanging places with neighboring molecules in the same monolayer; & 3. Transverse diffusion or “flip – flop” from one monolayer to another. For transverse diffusion to occur lipid head group which is polar must be charged and must move into hydrophobic interior of the bilayer. This movement requires energy in the form of ATP.  Rapid movements in the membranes is due to the presence of enzymes called phopsholipid translocators namely Flippases and Floppases, that catalyze the transverse diffusion of phospholipid molecules from one monolayer to another.  Flippases move the phospholipids from the outer monolayer to the cytoplasmic surface of plasma membrane.  Floppases move the phospholipids from the cytoplasmic surface of plasma membrane to the outer monolayer. 14-09-2024 Cell Morphology: External Cellular Components 19  Plasma Membrane Fluidity/Motion of Lipids  Lipids are not rigid/static structure.  In lipid bilayer, lipid molecules rotate freely around the long axis (rotational motion).  Lipids can also diffuse laterally around each leaflet.  The ability of lipids to traverse laterally and along the axis in lipid bilayer indicates that it can act as fluid.  The flip-flop of lipids might happen in several hours to days, and depends on the length of lipid molecule as well as its degree of unsaturation.  Membrane fluidity depends on two factors: 1. Temperature 2. Lipid Composition 14-09-2024 Cell Morphology: External Cellular Components 20  Plasma Membrane Fluidity/Motion of Lipids Effect of temperature on membrane fluidity  Increase in temperature increases the membrane fluidity (free-flowing, less viscous).  Decrease in temperature results in the membrane forming gel like structure The process of change in temperature and fluidity of plasma membrane is called Phase Transition. Temperature at which the transition occurs is called Transition Temperature. Effect of nature of fatty acids on membrane fluidity  Lipids with short/unsaturated fatty acyl chains undergo phase transition at lower temperature than lipids with long or saturated fatty acids. Short chains have less surface area, so the tendency of van der wall interaction is less, therefore can assume fluidic nature much easily.  Unsaturated fatty acids have kinks, thus tends to adopt a more random fluid state and form less Van der Wall interaction with other lipids. Thus, increased proportion of unsaturated to saturated fatty acids in the membrane increases fluidity of bilayer with reduced temperature. 14-09-2024 Cell Morphology: External Cellular Components 21  Plasma Membrane Fluidity/Motion of Lipids Effect of nature of fatty acids on membrane fluidity  Cholesterol is a major determinant of membrane fluidity.  At high temperature, cholesterol interferes with the movement of phospholipid fatty acid chains, making the membrane less fluid.  At low temperature, cholesterol interferes with the hydrophobic fatty acid chains and prevents the membrane from freezing. 14-09-2024 Cell Morphology: External Cellular Components 22  Bio-techniques used to Visualize/study Plasma Membrane 1. Hydropathy Plots: [Identify the number of alpha-helical protein residues in integral proteins] a) These plots is a means of representing hydrophobic regions and hydrophilic regions along the length of a transmembrane proteins (amino-acid sequence of protein). b) The plot has the amino-acid residues on the X-axis and degree of hydrophobicity and hydrophilicity on Y-axis. 2. Fluorescence Recovery after Photobleaching [FRAP]: The rapid lateral movement of membrane lipids can be visualized by the means of Fluorescence Microscopy through the use of Fluorescence Recovery after Photobleaching [FRAP]. 3. Freeze-fracture Technique: Freezing, fracturing and then subjected to vacuum (direct passage of ice to vapour) gives 3 dimensional view of the cell membrane with transmission electron microscope. 14-09-2024 Cell Morphology: External Cellular Components 23