Lodish H. - Molecular Cell Biology (5ed, Freeman, 2003) (794361), страница 98
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Thus ions, many low-molecular-weight precursors of cellular macromolecules, products of intermediarymetabolism, and small intracellular signaling molecules canpass from cell to cell through gap junctions.In nervous tissue, some neurons are connected by gapjunctions through which ions pass rapidly, thereby allowing very rapid transmission of electrical signals. Impulsetransmission through these connections, called electricalsynapses, is almost a thousandfold as rapid as at chemicalsynapses (Chapter 7).
Gap junctions are also present inmany non-neuronal tissues where they help to integrate the FIGURE 6-32 Molecular structure ofgap junctions. (a) Schematic model of agap junction, which comprises a cluster ofchannels between two plasma membranesseparated by a gap of about 2–3 nm. Bothmembranes contain connexonhemichannels, cylinders of six dumbbellshaped connexin molecules. Twoconnexons join in the gap between thecells to form a gap-junction channel,1.5–2.0 nm in diameter, that connects thecytosols of the two cells.
(b) Electrondensity of a recombinant gap-junctionchannel determined by electroncrystallography. Shown here are side viewsof the complete structure (top) and thesame structure with several chainsremoved to show the channel’s interior(center); on the bottom are perpendicularcross sections through the gap junctionwithin and between the membranebilayers.
There appear to be 24transmembrane helices per connexonhemichannel, consistent with each of thesix connexin subunits having four helices.The narrowest part of the channel is ≈1.5 nmin diameter. M membrane bilayer; E extracellular gap; C cytosol. [Part (b) fromV. M. Unger et al., 1999, Science 283:1176.]electrical and metabolic activities of many cells. In the heart,for instance, gap junctions rapidly pass ionic signals amongmuscle cells and thus contribute to the electrically stimulated coordinate contraction of cardiac muscle cells during abeat. As discussed in Chapter 13, some extracellular hormonal signals induce the production or release of small intracellular signaling molecules called second messengers(e.g., cyclic AMP and Ca2) that regulate cellular metabolism.
Because second messengers can be transferred betweencells through gap junctions, hormonal stimulation of onecell can trigger a coordinated response by that same cell andmany of its neighbors. Such gap junction–mediated signaling plays an important role, for example, in the secretionof digestive enzymes by the pancreas and in the coordinatedmuscular contractile waves (peristalsis) in the intestine. Another vivid example of gap junction–mediated transport isthe phenomenon of metabolic coupling, or metabolic cooperation, in which a cell transfers nutrients or intermediarymetabolites to a neighboring cell that is itself unable to synthesize them. Gap junctions play critical roles in the development of egg cells in the ovary by mediating the movementof both metabolites and signaling molecules between anoocyte and its surrounding granulosa cells as well as between neighboring granulosa cells.A current model of the structure of the gap junction isshown in Figure 6-32.
Vertebrate gap junctions are composed(a)(b)CMEMCConnexonhemichannelGapjunctionchannel2 nmCytosolIntercellular gap6.6 • Plant Tissuesof connexins, a family of structurally related transmembraneproteins with molecular weights between 26,000 and 60,000.A completely different family of proteins, the innexins, formsthe gap junctions in invertebrates. Each vertebrate hexagonal particle consists of 12 connexin molecules: 6 of the molecules are arranged in a connexon hemichannel—a hexagonalcylinder in one plasma membrane—and joined to a connexonhemichannel in the adjacent cell membrane, forming the continuous aqueous channel between the cells. Each connexinmolecule spans the plasma membrane four times; one conserved transmembrane helix from each subunit apparentlylines the aqueous channel.There are probably more than 20 different connexingenes in vertebrates, and different sets of connexins are expressed in different cell types.
Some cells express a single connexin; consequently their gap-junction channels arehomotypic, consisting of identical connexons. Most cells,however, express at least two connexins; these different proteins assemble into hetero-oligomeric connexons, which inturn form heterotypic gap-junction channels. This diversityin channel composition leads to differences in the permeability of channels to various molecules.
For example, channelsmade from a 43-kDa connexin isoform, Cx43, are more thana hundredfold as permeable to ADP and ATP as those madefrom Cx32 (32 kDa). Moreover, the permeability of gapjunctions can be altered by changes in the intracellular pHand Ca2 concentration, as well as by the phosphorylationof connexin, providing numerous mechanisms for regulating transport through them.The generation of mutant mice with inactivating mutations in connexin genes has highlighted the importance ofconnexins in a wide variety of cellular systems. For instance,Cx43-defective mice exhibit numerous defects including defective oocyte maturation due to decreased gap-junctionalcommunication between granulosa cells in the ovary.Mutations in several connexin genes are related tohuman diseases, including neurosensory deafness(Cx26 and Cx31), cataract or heart malformations(Cx43, Cx46, and Cx50), and the X-linked form of CharcotMarie-Tooth disease (Cx32), which is marked by progressivedegeneration of peripheral nerves.
❚KEY CONCEPTS OF SECTION 6.5Adhesive Interactions and Nonepithelial CellsMany nonepithelial cells have integrin-containing aggregates (e.g., focal adhesions, 3D adhesions, podosomes) thatphysically and functionally connect cells to the extracellular matrix and facilitate inside-out and outside-in signaling.■Integrins exist in two conformations that differ in theaffinity for ligands and interactions with cytosolic adapterproteins (see Figure 6-28).■231Dystroglycan, an adhesion receptor expressed by musclecells, forms a large complex with dystrophin, other adapterproteins, and signaling molecules (see Figure 6-29).
Thiscomplex links the actin cytoskeleton to the surrounding matrix, providing mechanical stability to muscle. Mutations invarious components of this complex cause different typesof muscular dystrophy.■Neural cell-adhesion molecules (CAMs), which belongto the immunoglobulin (Ig) family of CAMs, mediateCa2-independent cell–cell adhesion, predominantly inneural tissue and muscle.■The combinatorial and sequential interaction of severaltypes of CAMs (e.g., selectins, integrins, and ICAMs) is critical for the specific and tight adhesion of different types ofleukocytes to endothelial cells in response to local signalsinduced by infection or inflammation (see Figure 6-30).■Gap junctions are constructed of multiple copies of connexin proteins, assembled into a transmembrane channelthat interconnects the cytoplasm of two adjacent cells (seeFigure 6-32).
Small molecules and ions can pass throughgap junctions, permitting metabolic and electrical couplingof adjacent cells.■6.6 Plant TissuesWe turn now to the assembly of plant cells into tissues. The overall structural organization of plantsis generally simpler than that of animals. For instance, plants have only four broad types of cells, which inmature plants form four basic classes of tissue: dermal tissue interacts with the environment; vascular tissue transportswater and dissolved substances (e.g., sugars, ions); spacefilling ground tissue constitutes the major sites ofmetabolism; and sporogenous tissue forms the reproductiveorgans.
Plant tissues are organized into just four main organsystems: stems have support and transport functions; rootsprovide anchorage and absorb and store nutrients; leaves arethe sites of photosynthesis; and flowers enclose the reproductive structures. Thus at the cell, tissue, and organ levels,plants are generally less complex than most animals.Moreover, unlike animals, plants do not replace or repairold or damaged cells or tissues; they simply grow new organs. Most importantly for this chapter and in contrast withanimals, few cells in plants directly contact one anotherthrough molecules incorporated into their plasma membranes.
Instead, plant cells are typically surrounded by arigid cell wall that contacts the cell walls of adjacent cells(Figure 6-33). Also in contrast with animal cells, a plant cellrarely changes its position in the organism relative to othercells. These features of plants and their organization have determined the distinctive molecular mechanisms by whichtheir cells are incorporated into tissues. ❚232CHAPTER 6 • Integrating Cells into TissuesPrimarywallPectinCellulosemicrofibril50 nmHemicellulosePlasma membrane▲ FIGURE 6-33 Schematic representation of the cell wallof an onion. Cellulose and hemicellulose are arranged intoat least three layers in a matrix of pectin polymers.
The size ofthe polymers and their separations are drawn to scale. Tosimplify the diagram, most of the hemicellulose cross-linksand other matrix constituents (e.g., extensin, lignin) are notshown. [Adapted from M. McCann and K. R. Roberts, 1991, in C. Lloyd,ed., The Cytoskeletal Basis of Plant Growth and Form, Academic Press,p. 126.]The Plant Cell Wall Is a Laminate of CelluloseFibrils in a Matrix of GlycoproteinsThe plant cell wall is ≈0.2 m thick and completely coats theoutside of the plant cell’s plasma membrane. This structureserves some of the same functions as those of the extracellular matrix produced by animal cells, even though the twostructures are composed of entirely different macromoleculesand have a different organization.
Like the extracellular matrix, the plant cell wall connects cells into tissues, signals aplant cell to grow and divide, and controls the shape of plantorgans. Just as the extracellular matrix helps define theshapes of animal cells, the cell wall defines the shapes ofplant cells. When the cell wall is digested away from plantcells by hydrolytic enzymes, spherical cells enclosed by aplasma membrane are left. In the past, the plant cell wall wasviewed as an inanimate rigid box, but it is now recognized asa dynamic structure that plays important roles in controlling the differentiation of plant cells during embryogenesisand growth.Because a major function of a plant cell wall is to withstand the osmotic turgor pressure of the cell, the cell wall isbuilt for lateral strength.
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