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    which of the following best explains how the phospholipid bilayer of a transport vesicle contributes to cellular functions?

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    Transport from the ER through the Golgi Apparatus

    As discussed in Chapter 12, newly synthesized proteins enter the biosynthetic- secretory pathway in the ER by crossing the ER membrane from the cytosol. During their subsequent transport, from the ER to the Golgi apparatus and from the Golgi apparatus to the cell surface and elsewhere, these proteins pass through a series of compartments, where they are successively modified. Transfer from one compartment to the next involves a delicate balance between forward and backward (retrieval) transport pathways. Some transport vesicles select cargo molecules and move them to the next compartment in the pathway, while others retrieve escaped proteins and return them to a previous compartment where they normally function. Thus, the pathway from the ER to the cell surface involves many sorting steps, which continually select membrane and soluble lumenal proteins for packaging and transport—in vesicles or organelle fragments that bud from the ER and Golgi apparatus.

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    Molecular Biology of the Cell. 4th edition.

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    Transport from the ER through the Golgi Apparatus

    As discussed in Chapter 12, newly synthesized proteins enter the biosynthetic- secretory pathway in the ER by crossing the ER membrane from the cytosol. During their subsequent transport, from the ER to the Golgi apparatus and from the Golgi apparatus to the cell surface and elsewhere, these proteins pass through a series of compartments, where they are successively modified. Transfer from one compartment to the next involves a delicate balance between forward and backward (retrieval) transport pathways. Some transport vesicles select cargo molecules and move them to the next compartment in the pathway, while others retrieve escaped proteins and return them to a previous compartment where they normally function. Thus, the pathway from the ER to the cell surface involves many sorting steps, which continually select membrane and soluble lumenal proteins for packaging and transport—in vesicles or organelle fragments that bud from the ER and Golgi apparatus.

    In this section we focus mainly on the Golgi apparatus (also called the Golgi complex). It is a major site of carbohydrate synthesis, as well as a sorting and dispatching station for the products of the ER. Many of the cell's polysaccharides are made in the Golgi apparatus, including the pectin and hemicellulose of the cell wall in plants and most of the glycosaminoglycans of the extracellular matrix in animals (discussed in Chapter 19). But the Golgi apparatus also lies on the exit route from the ER, and a large proportion of the carbohydrates that it makes are attached as oligosaccharide side chains to the many proteins and lipids that the ER sends to it. A subset of these oligosaccharide groups serve as tags to direct specific proteins into vesicles that then transport them to lysosomes. But most proteins and lipids, once they have acquired their appropriate oligosaccharides in the Golgi apparatus, are recognized in other ways for targeting into the transport vesicles going to other destinations.

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    Proteins Leave the ER in COPII-coated Transport Vesicles

    To initiate their journey along the biosynthetic-secretory pathway, proteins that have entered the ER and are destined for the Golgi apparatus or beyond are first packaged into small COPII-coated transport vesicles. These transport vesicles bud from specialized regions of the ER called whose membrane lacks bound ribosomes. In most animal cells, ER exit sites seem to be randomly dispersed throughout the ER network.

    Originally it was thought that all proteins that are not tethered in the ER enter transport vesicles by default. However, it is now clear that packaging into vesicles that leave the ER can also be a selective process. Some cargo proteins are actively recruited into such vesicles, where they become concentrated. It is thought that these cargo proteins display exit (transport) signals on their surface that are recognized by complementary receptor proteins that become trapped in the budding vesicle by interacting with components of the COPII coat (Figure 13-17). At a much lower rate, proteins without such exit signals can also get packaged in vesicles, so that even proteins that normally function in the ER (so-called slowly leak out of the ER. Similarly, secretory proteins that are made in high concentrations may leave the ER without the help of sorting receptors.

    Figure 13-17

    The recruitment of cargo molecules into ER transport vesicles. By binding to the COPII coat, membrane and cargo proteins become concentrated in the transport vesicles as they leave the ER. Membrane proteins are packaged into budding transport vesicles (more...)

    The exit signals that direct proteins out of the ER for transport to the Golgi and beyond are mostly not understood. There is one exception, however. The ERGIC53 protein seems to serve as a receptor for packaging some secretory proteins into COPII-coated vesicles. Its role in protein transport was identified because humans who lack it owing to an inherited mutation have lowered serum levels of two secreted blood-clotting factors (Factor V and Factor VIII) and therefore bleed excessively. The ERGIC53 protein is a lectin that binds mannose and is thought to recognize this sugar on Factor V and Factor VIII proteins, thereby packaging the proteins into transport vesicles in the ER.

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    Only Proteins That Are Properly Folded and Assembled Can Leave the ER

    To exit from the ER, proteins must be properly folded and, if they are subunits of multimeric protein complexes, they may need to be completely assembled. Those that are misfolded or incompletely assembled are retained in the ER, where they are bound to chaperone proteins (see Chapter 6), such as or . The chaperones may cover up the exit signals or somehow anchor the proteins in the ER (Figure 13-18). Such failed proteins are eventually transported back into the cytosol where they are degraded by proteasomes (discussed in Chapter 12). This quality-control step is important, as misfolded or misassembled proteins could potentially interfere with the functions of normal proteins if they were transported onward. The amount of corrective action is surprisingly large. More than 90% of the newly synthesized subunits of the T cell receptor (discussed in Chapter 24) and of the acetylcholine receptor (discussed in Chapter 11), for example, are normally degraded in the cell without ever reaching the cell surface, where they function. Thus, cells must make a large excess of many protein molecules from which to select the few that fold and assemble properly.

    Source : www.ncbi.nlm.nih.gov

    Phospholipid Bilayer

    Phospholipid Bilayer

    Do phospholipid bilayers provide such continuity in the two-dimensional organizations of membranes or is the membrane an assembly of subunits consisting of proteins (Frey-Wyssling, 1955) or lipoproteins (Lucy, 1964;

    From: Structure and Function of Biological Membranes, 1971

    Related terms:

    PeptidePhospholipidPhospholipidsPhosphoproteinBilayer MembraneCell MembraneLipid Bilayer

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    High Pressure Studies Using NMR Spectroscopy

    Jiri Jonas, in Encyclopedia of Spectroscopy and Spectrometry (Third Edition), 2017

    Model Membranes

    Phospholipid bilayers and monolayers have important roles in nature as components of cell membranes and lipoproteins. In cell membranes, phospholipid bilayers constitute the permeability barriers between aqueous compartments. Aside from their structural interfacial roles, aggregated phospholipids modulate the functions of associated proteins and enzymes by direct binding and by physical effects. Therefore, synthetic phospholipid bilayers, in multilamellar or small bilayer vesicle forms, have become models for the study of the structural and dynamic properties of natural phospholipid aggregates.

    Since the 1970s, NMR methods, in particular 2H NMR, have been applied very effectively to investigate the biophysical properties of phospholipid bilayers. NMR experiments with high pressure and variable temperature measurements allow one to access diverse gel phases in phospholipid bilayers and to study the order and dynamics of phospholipids in bilayers as a function of volume changes.

    Table 3 gives illustrative examples of high pressure NMR studies of model membranes and indicates the type of information that can be obtained from these studies. Figure 10 shows the pressure–temperature phase diagram of deuterated dipalmitoylphosphatidylcholine, DPPC-d62, as determined by high pressure NMR techniques using direct measurement of the deuteron quadrupole splitting of the methyl group and first moment analysis.

    Table 3. Examples of NMR studies of the pressure effects on model membranesa

    System Experiment Result

    DPPCb Natural abundance 13C, T1, T2 Phase transitions

    DPPC-d62 DPPC-d62-TTCc 2H line shapes Phase diagram; order parameter; pressure reversal of the anaesthetic effect of tetracaine

    DPPC-TTC 31P line shapes, T1 Structure and dynamics of the head group; phase diagram

    DPPC-d2 (2,2); (9,9); (13,13) 2H line shapes, T1, T2 Order parameters; chain motions

    DPPC-d62-cholesterol 2H line shapes Phase diagram

    DPPC 1HT1 and T1ρ Lateral diffusion

    POPCd a

    Pressure range from 0.1 to 500 MPa.

    b

    DPPC=dipalmitoylphosphatidylcholine.

    c TTC=tetracaine. d

    POPC=palmitoyloleylphosphatidylcholine.

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    Figure 10. Pressure–temperature phase diagram of DPPC-d62.

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    Single Molecule Tools: Fluorescence Based Approaches, Part A

    Abhinav Nath, ... Elizabeth Rhoades, in Methods in Enzymology, 2010

    1 Introduction

    Phospholipid bilayer Nanodiscs (Bayburt and Sligar, 2009; Bayburt et al., 2002; Nath et al., 2007a; Ritchie et al., 2009) are an emerging model membrane system for the study of membrane-associated proteins. Nanodiscs consist of a phospholipid bilayer surrounded by a protein coat formed of membrane scaffold protein (MSP) and are derived from nascent (discoidal) high-density lipoprotein (HDL) particles. Nanodiscs are more stable and monodisperse than conventional model membranes such as liposomes, bicelles, and micelles, and are thus a very appealing model system for a range of biochemical and biophysical experiments with integral and peripheral membrane proteins. Given the importance of membrane proteins in so many biological and pharmacological questions, there has been an understandable interest in novel Nanodisc technology and a number of exciting developments in membrane protein biochemistry over the past few years (Alami et al., 2007; Boldog et al., 2006; Morrissey et al., 2008).

    Concurrent with the growing use of Nanodiscs, there has been a rise in the application of single-molecule fluorescence techniques to a range of biological problems, including movement of motor proteins (Park et al., 2007; Peterman et al., 2004), ribosome dynamics (Blanchard et al., 2004), and enzyme catalysis (Henzler-Wildman et al., 2007; Lu et al., 1998), that have provided fundamentally new mechanistic insights and a new appreciation for the role of stochasticity and nonlinear dynamics in a range of biological processes. Several groups have recently reported the application of single-molecule fluorescence to integral membrane protein incorporated in Nanodiscs (Nath et al., 2008b) or HDL particles (Kuszak et al., 2009; Whorton et al., 2007). In this chapter, we present detailed protocols from our published work, as well as new methods and results using Nanodiscs to study peripheral membrane-binding proteins, in the hope that this will prove useful to other investigators of membrane proteins.

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    Endocytosis

    Merri Lynn Casem BA, PhD, in Case Studies in Cell Biology, 2016

    Methods

    SUPER templates

    Phospholipid bilayers (SUPER templates) were formed on the surface of 5 μm silica beads by mixing the beads with liposomes. The phospholipids used in the construction of the liposomes were matched to the composition of the inner leaf of the cell membrane. Some of these phospholipids were labeled with the red fluorescent molecule RhPE, making it possible to visualize the artificial membrane that coated the silica beads. Changes in the location or intensity of fluorescence were markers for changes in the membrane surrounding the bead.

    Source : www.sciencedirect.com

    Lipid bilayer

    Lipid bilayer

    From Wikipedia, the free encyclopedia

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    This fluid lipid bilayer cross section is made up entirely of phosphatidylcholine.

    The three main structures phospholipids form in solution; the liposome (a closed bilayer), the micelle and the bilayer.

    The lipid bilayer (or phospholipid bilayer) is a thin polar membrane made of two layers of lipid molecules. These membranes are flat sheets that form a continuous barrier around all cells. The cell membranes of almost all organisms and many viruses are made of a lipid bilayer, as are the nuclear membrane surrounding the cell nucleus, and membranes of the membrane-bound organelles in the cell. The lipid bilayer is the barrier that keeps ions, proteins and other molecules where they are needed and prevents them from diffusing into areas where they should not be. Lipid bilayers are ideally suited to this role, even though they are only a few nanometers in width,[1] because they are impermeable to most water-soluble (hydrophilic) molecules. Bilayers are particularly impermeable to ions, which allows cells to regulate salt concentrations and pH by transporting ions across their membranes using proteins called ion pumps.

    Biological bilayers are usually composed of amphiphilic phospholipids that have a hydrophilic phosphate head and a hydrophobic tail consisting of two fatty acid chains. Phospholipids with certain head groups can alter the surface chemistry of a bilayer and can, for example, serve as signals as well as "anchors" for other molecules in the membranes of cells.[2] Just like the heads, the tails of lipids can also affect membrane properties, for instance by determining the phase of the bilayer. The bilayer can adopt a solid gel phase state at lower temperatures but undergo phase transition to a fluid state at higher temperatures, and the chemical properties of the lipids' tails influence at which temperature this happens. The packing of lipids within the bilayer also affects its mechanical properties, including its resistance to stretching and bending. Many of these properties have been studied with the use of artificial "model" bilayers produced in a lab. Vesicles made by model bilayers have also been used clinically to deliver drugs

    The structure of Biological membranes typically includes several types of molecules in addition to the phospholipids comprising the bilayer. A particularly important example in animal cells is cholesterol, which helps strengthen the bilayer and decrease its permeability. Cholesterol also helps regulate the activity of certain integral membrane proteins. Integral membrane proteins function when incorporated into a lipid bilayer, and they are held tightly to the lipid bilayer with the help of an annular lipid shell. Because bilayers define the boundaries of the cell and its compartments, these membrane proteins are involved in many intra- and inter-cellular signaling processes. Certain kinds of membrane proteins are involved in the process of fusing two bilayers together. This fusion allows the joining of two distinct structures as in the acrosome reaction during fertilization of an egg by a sperm, or the entry of a virus into a cell. Because lipid bilayers are fragile and invisible in a traditional microscope, they are a challenge to study. Experiments on bilayers often require advanced techniques like electron microscopy and atomic force microscopy.

    Contents

    1 Structure and organization

    1.1 Cross section analysis

    1.2 Asymmetry

    1.3 Phases and phase transitions

    1.4 Surface chemistry

    2 Biological roles

    2.1 Containment and separation

    2.2 Signaling

    3 Characterization methods

    3.1 Electrical measurements

    3.2 Fluorescence microscopy

    3.3 Electron microscopy

    3.4 Nuclear magnetic resonance spectroscopy

    3.5 Atomic force microscopy

    3.6 Dual polarisation interferometry

    3.7 Quantum chemical calculations

    4 Transport across the bilayer

    4.1 Passive diffusion

    4.2 Ion pumps and channels

    4.3 Endocytosis and exocytosis

    4.4 Electroporation 5 Mechanics 6 Fusion 7 Model systems

    8 Commercial applications

    9 History 10 See also 11 References 12 External links

    Structure and organization[edit]

    When phospholipids are exposed to water, they self-assemble into a two-layered sheet with the hydrophobic tails pointing toward the center of the sheet. This arrangement results in two “leaflets” that are each a single molecular layer. The center of this bilayer contains almost no water and excludes molecules like sugars or salts that dissolve in water. The assembly process is driven by interactions between hydrophobic molecules (also called the hydrophobic effect). An increase in interactions between hydrophobic molecules (causing clustering of hydrophobic regions) allows water molecules to bond more freely with each other, increasing the entropy of the system[]. This complex process includes non-covalent interactions such as van der Waals forces, electrostatic and hydrogen bonds.

    Cross section analysis[edit]

    Schematic cross sectional profile of a typical lipid bilayer. There are three distinct regions: the fully hydrated headgroups, the fully dehydrated alkane core and a short intermediate region with partial hydration. Although the head groups are neutral, they have significant dipole moments that influence the molecular arrangement.[3]

    Source : en.wikipedia.org

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