Plasma Membrane PDF

PLASMA MEMBRANE STRUCTURE AND FUNCTIONS  CELL MEMBRANE The plasma membrane - refers both to the membrane that surrounds cells and also to the membranes that surround organelles within the cell. The functions of the plasma membrane are Compartmentalization (...

PLASMA MEMBRANE STRUCTURE AND FUNCTIONS  CELL MEMBRANE The plasma membrane - refers both to the membrane that surrounds cells and also to the membranes that surround organelles within the cell. The functions of the plasma membrane are Compartmentalization (separate organelles from other stuff) give the cell shape Barrier (keep some things out and others in) Gatekeeper (do let some things through but not others) Monitoring outside signals (receptors on the membrane, signal to other proteins inside the cell)  Plasma Membrane DYNAMIC - The flexibility& its capacity for expansion allow the cell to grow, change its shape and move Enlarges in area by adding new membranes without loosing continuity. It can reseal , if pierced.  3-4 nm thick and 7-8 nm width A glycerol linker. A phosphate group. Two fatty acid chains. The polar head group faces outward into the aqueous intracellular or extracellular space, The hydrophobic interior of the membrane is the fatty acid chains. The phospholipids are named after their head groups  LIPIDS IN MEMBRANE phosphatidylserine (PS), phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylinositol (PI). The phospholipids components of cell membrane are of two kinds i) neutral phospholipids and ii) acidic phospholipids Neutral phospholipids such as phosphatidylcholine, phosphatidylethanolamine and sphingomyelin have no net charge at neutral pH and they tend to pack tightly in the bilayer. Acidic phospholipids such as phosphatidylinositol, phosphatidylserne, cardiolipin, phosphatidylgcerol, sulpholipds are negatively charged and in the membrane are associated principally with proteins by way of lipid-protein interactions.  THE LIPID BILAYER Phosphoglycerides, a principal class of phospholipids, fatty acyl side chains are esterified to two of the three hydroxyl groups in glycerol, and the third hydroxyl group is esterified to phosphate Phosphatidylcholine –has small molecule choline attached to a phosphate group as its hydrophilic head -The phosphate group is esterified to a hydroxyl group on choline Instead of choline, alcohols such as ethanolamine, serine, and the sugar derivative inositol are linked to the phosphate in other phosphoglycerides The negative charge on the phosphate as well as the charged groups or hydroxyl groups on the alcohol esterified to it interact strongly with water. Both of the fatty acyl side chains in a phosphoglyceride may be saturated or unsaturated, or one chain may be saturated and the other unsaturated.  Sphingolipids-are a class of lipids containing a backbone of sphingoid bases, a set of aliphatic amino alcohols that includes sphingosine cholesterol -in animal cells only Glycolipids - glycolipids –amphipathic but have one or more sugars linked with polar head-cerebrosides and gangliosides Cholesterol- polar head formed by OH group, on polar sterol ring and a non polar hydrocarbon tail.  cholesterol, regulates the fluidity of the cell membrane in animal cells. When there is less cholesterol, membranes become more fluid, but also more permeable to molecules. The amount of cholesterol in the membrane helps maintain its permeability - the right amount of molecules can enter the cell at a time, not too many or too few. Cholesterol affects fluidity: at body temperature it lessens fluidity by restraining the movement of phospholipids, at (a) Shape of cholesterol molecule- (b) how it fill the gaps in bilayer colder temperatures it adds fluidity by not allowing phospholipids to pack close together Cellular membranes are laterally heterogeneous and consist of transient and dynamic domains with varying properties. These domains prominently include ordered lipid-driven domains that are referred to as lipid (or membrane) rafts. These microdomains(OM) are enriched in cholesterol and sphingolipids. They change in composition as individual lipids and proteins move in and out. play an important role in many cellular processes including signal transduction, membrane trafficking, cytoskeletal organization, and pathogen entry. Membrane rafts potentially have crucial physiological roles across cell types that range from immune cells to cancer cells. Membrane domains are conserved throughout the domains of life, which supports their functional importance in biological systems.  Proteins are the second major component of PM Represent about a third of the proteins in living organisms. Integral membrane protein - is permanently anchored or part of the membrane, Peripheral membrane protein - only temporarily attached to the lipid bilayer or to other integral proteins Lipid-anchored proteins 1) Rapidly freeze specimen 2) Use special knife to cut membrane in half 3) Apply a carbon + platinum coating to the surface 4) Use scanning electron microscope to see the surface  Integral proteins Integral proteins that penetrates the lipid bilayer. They are also referred as transmembrane proteins They pass entirely through the lipid bilayer and have domains that protrude from both the extracellular and cytoplasmic sides of the membrane. Genomic sequencing studies suggest that integral proteins constitute 20-30% of all encoded proteins. Because of their hydrophobic transmembrane domains, integral membrane proteins are difficult to isolate in a soluble form. Removal of these proteins from the membrane normally requires the use of a detergent, such as the ionic (charged) detergent (SDS) sodium dodecyl sulfate (which denatures proteins). Non-ionic (uncharged) detergent Triton X-100(which generally does not alter a protein’s tertiary structure).  MEMBRANE PROTEINS According to their their relationship with the bilayer, integral membrane protein can be integral polytopic proteins. Integral polytopic proteins are also known as “transmembrane proteins” which can span across the membrane. These integral membrane proteins may have different transmembrane topology which refers to orientations (locations of N- and C-termini) of membrane-spanning segments with respect to the inner or outer sides of the biological membrane occupied by the protein.  MEMBRANE PROTEINS Integral membrane proteins - permanently attached to the membrane,. Integral monotopic proteins are one type of integral membrane proteins that are attached to only one side of the membrane and do not span the whole way across. There are 4 types of interaction between Integral monotopic membrane protein and cell membranes: by an amphipathicα-helix parallel, by a hydrophobic loop, by a covalently bound membrane lipid and electrostatic or ionic interaction with membrane lipids (No. 4, 5, 6,7). 1. A single transmembrane α-helix (bitopic membrane protein). 2. A polytopic transmembrane α-helical protein. 3. A polytopic transmembrane β- sheet protein. 4. Interaction by an amphipathic α-helix parallel to the membrane plane (in-plane membrane helix). 5. Interaction by a hydrophobic loop. 6. Interaction by a covalently bound membrane lipid. 7. Ionic or electrostatic interactions with membrane lipids.  MEMBRANE PROTEINS Integral membrane proteins - permanently attached to the membrane,. Membrane Proteins Associate with the Lipid Bilayer in Different Ways A Polypeptide Chain Crosses the Lipid Bilayer All membrane proteins have a unique orientation in lipid bilayer  TM proteins -have one of two structural architectures: helix bundle proteins, which are present in all types of biological membranes; Ex -inner membranes of bacterial cells or the plasma membrane of eukaryotes, and sometimes in the outer membranes. Glycophorins, PRC, Bacteriorhodopsin, opsin in eye. Beta barrel proteins- found only in outer membranes of Gram-negative bacteria Backbone of a polypeptide chain is hydrophilic lipid-rich cell walls of a few Gram-positive bacteria outer membranes of mitochondria and chloroplasts. Nucleoporins …Large enough to allow passive diffusion. Selective for one type of molecule or group of one type Transmembrane single crossing- receptors for extracellular signals Transmembrane hydrophilic pore –multiple amphipathic alpha helices ( 5 transmembrane alpha helices form a water filled channel)  Bacteriorhodopsin- Transmembrane protein Small protein –found in PM of halobacterium halobium, an Achaean living in salt marshes – Protein acts as membrane transport protein that pumps protons out of cells ,which requires energy Each bacteriorhodopsin gets energy from directly from Sunlight It contain a single light absorbing molecule – RETINAL –non protein It gives the protein and bacterium deep purple color This small hydrophobic molecule is covalently attached to one of the 7 transmembrane alpha helices  Retinal absorbs photon of light, changes shape, cause protein in bilayer to undergo small conformational changes, resulting in transfer of one proton from inside to outside Retinal gets regenerated by taking up a proton from cytosol. Protein returns to its original conformation, repeat the cycle Overall movement is one proton –inside to outside 1000’s bacteriorhodopsin molecules pump protons out of cell Generate a concentration gradient of protons across PM.Cell uses this proton to store energy Bacteria –transmembrane protein actively moves small organic /inorganic ions into and out of cells  Peripheral membrane proteins Peripheral membrane proteins not embedded into the lipid bilayer but they are loosely associated with the membrane surface. Peripheral proteins- located entirely outside of the lipid bilayer, on either the cytoplasmic or the extracellular side and are associated with the surface of the membrane by non-covalent bonds. Attached either to integral proteins or to phospholipids. These may be removed from the membrane, or solubilized, by mild treatment, proteinsdissociate following treatment with a polar reagent, such as a solution with an elevated pH or high salt concentrations. Unlike integral membrane proteins, peripheral membrane proteins do not stick into the hydrophobic core of the membrane  Peripheral membrane proteins are temporarily attached either to the lipid bilayer or to integral proteins by a combination of hydrophobic, electrostatic, and other non-covalent interactions. Dissociate following treatment with a polar reagent, such as a solution with an elevated pH or high salt concentrations. Cytoskeletal proteins SPECTRIN & ACTIN in erythrocytes Enzyme Protein kinase C –PKC shuttles between cytosol & cytosolic face of PM-signal transduction ECM protein on outer surface of PM  MEMBRANE PROTEINS Membrane Proteins Associate with the Lipid Bilayer in one of these different ways- serve various functions  There are six functional forms of integral membrane proteins Pumps -transport ions (Na+, K+) activity across the membrane. Channels -transport certain substances passively across the membrane. Receptors -allow binding and recognition of specific substances (ligands) e.g. hormone on the extracellular surface of the membrane. Enzymes possess various enzymatic activities, for example, ATP synthase of the inner mitochondrial membrane and some types of digestive enzymes in the small intestine. Linkers serve to anchor the intracellular cytoskeleton to the extracellular matrix. Structural proteins form junctions between neighbouring cells.  THE LIPID BILAYER-FLUIDITY Fluidity –ease with which lipid move within the plane of bilayer Important for membrane function -has to be maintained within certain limits How fluid a lipid bilayer is at a given temperature ? It depends on its phospholipid composition - Nature of hydrocarbon tails The closer and more regularly packed hydrocarbon tails – more viscous –less fluid bilayer Two major properties of hydrocarbon tails affect how tightly packed they are in Lipid bilayer (i)length and (ii)no of double bonds they contain Shorter chain length- reduces interaction, increase fluidity Double bond – creates small kink – loosely packed aggregates –fluid difficult to pack against one another. LIPID BILAYERS containing a large proportions of unsaturated FA tails are more fluid than those with lower proportions Sterol content  Bacteria and yeast cells –adapt to varying temperatures Length and unsaturation of FA in bilayer -constantly adjusted to maintain membrane at a relatively constant fluidity At higher temperatures – cell makes lipids that are longer and with fewer double bonds Animal cells – fluidity is maintained by inclusion of cholesterols Small and rigid molecule ,stiffens bilayer ,makes membrane less fluid and less permeable.  Membrane fluidity is important Proteins diffuse and interact with each other- otherwise membrane will be stagnant Cell signaling or other physiological functions will be hampered Diffusion of membrane lipids and proteins (synthesis to site of insertion)  Studying membrane fluidity The fluidity of the membrane can be studied with fluorescence recovery after photobleaching (FRAP). In this protocol, a molecule of interest is fluorescently tagged – for instance, a membrane protein is tagged with a fluorescently labelled antibody or fused with green fluorescent protein, or an amphipathic molecule is tagged with a fluorophore. ‘bleach’ part of the membrane with a laser which exhausts the fluorescent properties of the fluorescent molecule in that patch. Watch as still-fluorescent molecules from elsewhere in the membrane diffuse into the bleached patch. The main goal here is to see how ‘mobile’ the protein being studied. Generally, membrane proteins are less mobile than the phospholipids because they are so much larger. if the protein is anchored to the cytoskeleton, it won’t move at all, so the bleached patch will recover very slowly if at all. To control for the effects of membrane fluidity you can also tag the phospholipids themselves and compare the mobility of the protein of interest to that of the phospholipids. Membrane Assembly - begins in the ER New phospholipids are manufactured by enzymes bound to cytosolic surface of ER, using free FA as substrates Newly made phospholipids is deposited exclusively in the cytosolic half of the bilayer Certain Phospholipids Are Confined to One Side of the Membrane- most CM are assymetrical Two halves of the membrane has different set of phospholipids New membrane emerge from ER –with evenly scrambled set of Phospholipids Asymmetry arise from –GOLGI Membrane contain enzymes – flippases Flippase remove phospholipids from the outer leaflet to the inner leaflet facing cytosol Floppases move phospholipids in the opposite direction, particularly the choline derived phospholipids phospatidylcholine and sphingomyelin.  In order to maintain the charge gradient across the membrane, flippases predominantly transport phosphatidylserine and to a lesser extent phosphatidylethanolamine. Floppases also mediate cholesterol transport from the intracellular monolayer to the extracellular monolayer Asymmetry is preserved as membrane bud off from one organelle and fuse with another or PM Membranes are structurally and functionally asymmetric. The outer and inner surfaces of all known biological membranes have different components and functions. Difference in composition of lipids and proteins Difference in positioning and orientation of membrane proteins different enzymatic activities of inner and outer surfaces Asymmetry is crucial to the functioning of the membrane – Example - pump that regulates the concentration of Na+ and K+ ions in cells. Used to establish electrochemical gradient- The Na+-K+ pump is oriented so that it pumps Na+ out of the cell and K+ into it. The pump can work only if the pump orientation is correct. Furthermore, ATP must be on the inside of the cell to drive the pump. These proteins are placed in the membrane in the asymmetric fashion following their synthesis.  Membrane proteins have a unique orientation because they are synthesized and inserted into the membrane in an asymmetric manner. This absolute asymmetry is preserved – for longer periods Reasons: membrane proteins do not rotate from one side of the membrane to the other or FLIP FLOP (too energetically unfavorable ) because membranes are always synthesized by the growth of preexisting membranes- that are asymmetric Lipids, too, are asymmetrically distributed as a consequence of their mode of biosynthesis, but this asymmetry is usually not absolute (because they FLIP FLOP), except for glycolipids. In the red-blood-cell membrane, sphingomyelin and phosphatidyl choline are preferentially located in the outer leaflet of the bilayer whereas phosphatidyl ethanolamine and phosphatidyl serine are located mainly in the inner leaflet.The phosphatidyl inositol are also located on the cytosolic side of the bilayer. Cholesterol is distributed evenly throughout the two monolayers.  Although most phospholipids are neutral at physiologic pH, phosphatidylserine and phosphatidylinositol have a net negative charge at physiologic pH. Being present predominately in the inner leaflet, these two lipids generate a significant difference in charge between the two leaflets of the lipid bilayer. This generates a functionally relevant asymmetry in the membrane Membrane lipid asymmetry is important for signal transduction. Phosphatidyl serine is a binding partner for signaling proteins such as protein kinase C. The appearance of phosphatidyl serine on the outer leaflet of the cell membrane is an indication of a loss of membrane integrity Extracellular expression of phosphatidyl serine targets the cell for engulfment by macrophages and is widely used as a diagnostic marker for apoptosis. Maintaining membrane lipid asymmetry is highly important for cell homeostasis. DISTINCT INSIDE - OUTSIDE PM – non cytosolic –faced exterior Cytosolic – interior Orientation is conserved, not only PL but any protein inserted Vesicle with a membrane protein buds of from Golgi fusing with PM Orientation of membrane lipid and protein is preserved during the process GLYCOPROTEINS – attach sugar facing non cytosolic face GLYCOLIPIDS- distributed in CM Only in non cytosolic half –Sugar groups face cell exterior & Form part of continuous coat of carbohydrate layer Surround and protect animal cells Acquire sugar groups from Golgi.Enzyme engineering this chemical modifications are confined there Enzymes add sugar molecules to the non cytosolic half of the bilayer. once formed trapped in bilayer - No flippases Sugar on outside Phosphatidylcholine –red Sphingomyelin-brown –concentrated in noncytosolic layer. PSerine –light green, P-ethanolamine-yellow –cytosolic Phosphoinositol- dark green –cell signalling Glycolipids – blue  MEMBRANE PROTEINS Detergents -Small amphipathic lipid molecules –only one FA tail Tend to aggregate as small clusters in water – MICELLES Detergent Interfere with protein but leave lipid bilayer intact Other proteins –peripheral proteins released with gentle extraction procedure  Detergents with great excess are mixed with membranes Hydrophobic end of detergent interact with membrane spanning hydrophobic region of transmembrane protein and hydrophobic tails of PL Disrupting lipid bilayer and separating protein The other end of detergent molecule is hydrophilic –membrane protein comes into solution as protein –detergent complex Detergent solubilizes PL Separated for further analysis  PM is reinforced by Underlying cortex Cell membrane is extremely thin and fragile- strengthened and supported by framework of proteins attached via transmembrane proteins. Plants, bacteria and yeast-mechanical properties are conferred by rigid cell wall Animal cells – PM is stabilized by meshwork of fibrous proteins –cell cortex The cell cortex, or actin cortex or actomyosin cortex, is a specialized layer of cytoplasmic protein on the inner face of the plasma membrane of the cell periphery It functions as a modulator of plasma membrane behavior and cell surface properties. In most eukaryotic cells, the cortex is an actin-rich network consisting of Filamentous (F)- actin filaments, myosin motors, and actin-binding proteins  The actomyosin cortex is attached to the cell membrane via membrane-anchoring proteins ERM-(Ezrin,Radixin,Moesin) plays a central role in cell shape control. The protein constituents of the cortex undergo rapid turnover, making the cortex both mechanically rigid and highly plastic- properties essential to its function In mitosis, F-actin and myosin II form a highly contractile and uniform cortex - drive mitotic cell rounding. The surface tension produced by the actomyosin cortex activity generates intracellular hydrostatic pressure capable of displacing surrounding objects to facilitate rounding. In cytokinesis - plays a central role by producing a myosin-rich contractile ring to constrict the dividing cell into two daughter cells PM is reinforced by Underlying cortex  PM is reinforced by Underlying cortex Well studied cortex is in Human RBC- Cells are small Cortex – simple n regular main component of cortex is long thin flexible rod Protein – SPECTRIN - linked to transmembrane protein Provide support and maintain biconcave shape. Humans having genetic abnormalities in spectrin structure –anemic hereditary defects of the erythrocyte ,sometimes Red cells abnormally fragile Cortex in other animal cells is rich in actin and myosin ,much more complex.  Cellular proteins vary in biological function - A membrane may contain hundreds of different proteins depending on the cell type and the particular organelle within the cell, The parts of membrane proteins that interact with other cells or with extracellular substances project outward into the extracellular space whereas those parts of membrane proteins that interact with cytoplasmic molecules, project into the cytosol. Epithelial cells – lining small intestine Membrane facing intestinal lumen – rich in glycoprotein Opposite membrane – sodium pump Liver cells – PM of Parenchymal liver cell – junction with 3 different neighboring cells  THE LIPID BILAYER The Lipid Bilayer Is a flexible Two-dimensional Fluid-individual lipids can move around exchange places within the adjacent molecules within the monolayer. The Fluidity - Depends on Its Composition, studied using synthetic bilayers(no proteins) There is Spontaneous aggregation of amphipathic lipid molecules in water. Pure phospholipids form close spherical vesicle –liposomes,when added to water. Liposomes are composed of a lipid bilayer separating an aqueous internal compartment from the bulk aqueous phase Micelles are closed lipid monolayers with a fatty acid core and polar surface, or polar core with fatty acids on the surface Micelles are spherical amphiphilic structures - have a hydrophobic core and a hydrophilic shell. The hydrophilic shell makes the micelle water soluble that allows for intravenous delivery while the hydrophobic core carries a payload of drug for therapy  THE LIPID BILAYER phospholipids spontaneously form bilayers in aqueous environment Tend to close on themselves to form sealed compartments Self assembly and self sealing properties of membrane lipids in the form of lipid micelles –suggest lipid bilayer is fluid in nature  THE LIPID BILAYER LIPID BILAYER forms – energetically most favorable. Water facing (both surfaces of Bilayer) and water shielding Bilayer self sealing- same forces help. Any tear in sheet will create a free edge that is exposed to water- energetically unfavorable Molecules of bilayer spontaneously rearrange to eliminate the free edge Small tear- spontaneous rearrangement will exclude water molecules - repair of bilayer, restoring single continuous sheet Large tear - sheet fold on itself and break into separate closed vesicles Free edges are quickly eliminated Bend and reseal –forming a boundary around a closed space