H. Lodish - Molecular Cell Biology (5ed, Freeman, 2003) (796244), страница 64
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Phospholipidsin the cytosolic leaflet are resistant to hydrolysis by phospholipases added to the external medium because the enzymes cannot penetrate to the cytosolic face of the plasmamembrane.How the asymmetric distribution of phospholipids inmembrane leaflets arises is still unclear. In pure bilayers,phospholipids do not spontaneously migrate, or flip-flop,from one leaflet to the other. Energetically, such flip-floppingis extremely unfavorable because it entails movement of thepolar phospholipid head group through the hydrophobic interior of the membrane.
To a first approximation, the asym-RPolar head groupDOOPOO3CH22CH1CH2COOOA1155CC(CH2)nCH3A2O(CH2)nCH3▲ FIGURE 5-9 Specificity of phospholipases. Each type ofphospholipase cleaves one of the susceptible bonds shown inred. The glycerol carbon atoms are indicated by small numbers.In intact cells, only phospholipids in the exoplasmic leaflet of theplasma membrane are cleaved by phospholipases in thesurrounding medium. Phospholipase C, a cytosolic enzyme,cleaves certain phospholipids in the cytosolic leaflet of theplasma membrane.metry in phospholipid distribution results from the vectorialsynthesis of lipids in the endoplasmic reticulum and Golgi.Sphingomyelin is synthesized on the luminal (exoplasmic)face of the Golgi, which becomes the exoplasmic face of theplasma membrane.
In contrast, phosphoglycerides are synthesized on the cytosolic face of the ER membrane, which istopologically identical with the cytosolic face of the plasmamembrane (see Figure 5-4). Clearly, this explanation doesnot account for the preferential location of phosphatidylcholine in the exoplasmic leaflet.
Movement of this phosphoglyceride and perhaps others from one leaflet to the otherin some natural membranes is catalyzed by certain ATPpowered transport proteins called flippases discussed inChapters 7 and 18.The preferential location of lipids to one face of the bilayer is necessary for a variety of membrane-based functions.For example, the head groups of all phosphorylated forms ofphosphatidylinositol face the cytosol. Certain of them arecleaved by phospholipase C located in the cytosol; this enzyme in turn is activated as a result of cell stimulation bymany hormones.
These cleavages generate cytosol-solublephosphoinositols and membrane-soluble diacylglycerol. Aswe see in later chapters, these molecules participate in intracellular signaling pathways that affect many aspects of cellular metabolism. Phosphatidylserine also is normally mostabundant in the cytosolic leaflet of the plasma membrane.In the initial stages of platelet stimulation by serum, phosphatidylserine is briefly translocated to the exoplasmic face,presumably by a flippase enzyme, where it activates enzymesparticipating in blood clotting.156CHAPTER 5 • Biomembranes and Cell ArchitectureGM1PLAPRaft(a)YYCholera toxinAntibodiesYCopatchTfRCholera toxinYY(b)YAntibodiesSeparate patches▲ EXPERIMENTAL FIGURE 5-10 Some membrane lipidsand proteins colocalize in lipid rafts.
The results of biochemicalstudies suggested that GM1, a glycosphingolipid, and placentalalkaline phosphatase (PLAP), a lipid-anchored membrane protein,aggregate together into lipid rafts, whereas the transferrinreceptor (TfR), which traverses the entire membrane, does not.To locate these components in the intact plasma membrane,cells were treated with fluorescence-labeled cholera toxin(green), which cross-links closely spaced GM1 molecules, andwith fluorescence-labeled antibodies (red) specific for eitherPLAP or TfR. Each antibody can cross-link closely spacedCholesterol and Sphingolipids Cluster withSpecific Proteins in Membrane MicrodomainsThe results of recent studies have challenged the long-heldbelief that lipids are randomly mixed in each leaflet of a bilayer.
The first hint that lipids may be organized within theleaflets was the discovery that the residues remaining afterthe extraction of plasma membranes with detergents containtwo lipids: cholesterol and sphingomyelin. Because these twolipids are found in more ordered, less fluid bilayers, researchers hypothesized that they form microdomains, termedlipid rafts, surrounded by other more fluid phospholipidsthat are easily extracted by detergents.Biochemical and microscopic evidence supports the existence of lipid rafts in natural membranes. For instance, fluorescence microscopy reveals aggregates of lipids andraft-specific proteins in the membrane (Figure 5-10). Therafts are heterogeneous in size but are typically 50 nm indiameter.
Rafts can be disrupted by methyl--cyclodextrin,which depletes the membrane of cholesterol, or by antibiotics, such as filipin, that sequester cholesterol; such findings indicate the importance of cholesterol in maintaining themolecules of the protein that it recognizes. Cross-linking causesthe proteins or lipids to form larger patches that can be detectedby fluorescence microscopy (see Figure 5-42). (a) Micrograph of acell treated with toxin and with anti-PLAP antibody shows GM1and PLAP colocalized in the same patches (yellow). Thiscopatching suggests that both GM1 and PLAP are present in lipidrafts that coalesce in the presence of the cross-linking reagents.(b) Micrograph of a cell treated with toxin and with anti-TfRantibody shows that GM1 and TfR reside in separate patches(i.e., red and green), indicating that TfR is not a raft-residentprotein.
[Micrographs from T. Harder et al., 1998, J. Cell Biol. 141:929.]integrity of these rafts. Besides their enrichment by cholesterol and sphingolipids, lipid rafts are enriched for manytypes of cell-surface receptor proteins, as well as many signaling proteins that bind to the receptors and are activatedby them. These lipid–protein complexes can form only in thetwo-dimensional environment of a hydrophobic bilayer and,as discussed in later chapters, they are thought to facilitatethe detection of chemical signals from the external environment and the subsequent activation of cytosolic events.KEY CONCEPTS OF SECTION 5.1Biomembranes: Lipid Compositionand Structural OrganizationThe eukaryotic cell is demarcated from the external environment by the plasma membrane and organized intomembrane-limited internal compartments (organelles andvesicles).■The total surface area of internal membranes far exceedsthat of the plasma membrane.■5.2 • Biomembranes: Protein Components and Basic FunctionsThe phospholipid bilayer, the basic structural unit of allbiomembranes, is a two-dimensional lipid sheet with hydrophilic faces and a hydrophobic core, which is impermeable to water-soluble molecules and ions (see Figure 5-2).■Certain proteins present in biomembranes make themselectively permeable to water-soluble molecules and ions.■The primary lipid components of biomembranes are phosphoglycerides, sphingolipids, and steroids (see Figure 5-5).■Most lipids and many proteins are laterally mobile inbiomembranes.■Different cellular membranes vary in lipid composition(see Table 5-1).
Phospholipids and sphingolipids are asymmetrically distributed in the two leaflets of the bilayer,whereas cholesterol is fairly evenly distributed in bothleaflets.■Natural biomembranes generally have a fluidlike consistency. In general, membrane fluidity is decreased bysphingolipids and cholesterol and increased by phosphoglycerides. The lipid composition of a membrane also influences its thickness and curvature (see Figure 5-8).■Lipid rafts are microdomains containing cholesterol,sphingolipids, and certain membrane proteins that form inthe plane of the bilayer.
These aggregates are sites for signaling across the plasma membrane.■5.2 Biomembranes: ProteinComponents and Basic FunctionsMembrane proteins are defined by their location within or atthe surface of a phospholipid bilayer. Although every biological membrane has the same basic bilayer structure, theproteins associated with a particular membrane are responsible for its distinctive activities. The density and complement of proteins associated with biomembranes vary,depending on cell type and subcellular location. For example, the inner mitochondrial membrane is 76 percent protein;the myelin membrane, only 18 percent.
The high phospholipid content of myelin allows it to electrically insulate anerve cell from its environment. The importance of membrane proteins is suggested from the finding that approximately a third of all yeast genes encode a membrane protein.The relative abundance of genes for membrane proteins iseven greater in multicellular organisms in which membraneproteins have additional functions in cell adhesion.The lipid bilayer presents a unique two-dimensional hydrophobic environment for membrane proteins. Some proteins are buried within the lipid-rich bilayer; other proteinsare associated with the exoplasmic or cytosolic leaflet of thebilayer.
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