H. Lodish - Molecular Cell Biology (5ed, Freeman, 2003) (796244), страница 72
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Monomeric actin subunits assemble into microfilaments; dimeric subunits composed of- and -tubulin polymerize into microtubules. Unlike microfilaments and microtubules, which are assembled fromone or two proteins, intermediate filaments are assembledfrom a large diverse family of proteins. The most commonintermediate filaments, found in the nucleus, are composedof lamins. Intermediate filaments constructed from otherproteins are expressed preferentially in certain tissues: for example, keratin-containing filaments in epithelial cells,desmin-containing filaments in muscle cells, and vimentincontaining filaments in mesenchymal cells. FIGURE 5-29 Comparison of the three types offilaments that form the cytoskeleton. (a) Diagram of the basicstructures of an actin filament (AF), intermediate filament (IF),and microtubule (MT).
The beadlike structure of an actin filamentshows the packing of actin subunits. Intermediate filamentsubunits pack to form ropes in which the individual subunits aredifficult to distinguish. The walls of microtubules are formed fromprotofilaments of tubulin subunits. (b) Micrograph of a mixture ofactin filaments, microtubules, and vimentin intermediatefilaments showing the differences in their shape, size, andflexibility. Purified preparations of actin, tubulin, and vimentinsubunits were separately polymerized in a test tube to form thecorresponding filaments.
A mixture of the filaments was appliedto a carbon film on a microscope grid and then rinsed with adilute solution of uranyl acetate (UC), which surrounds but doesnot penetrate the protein (c). Because uranyl acetate is a heavymetal that easily scatters electrons, areas of the microscope gridoccupied by protein produce a “negative” image in metal filmwhen projected onto a photographic plate, as seen in part (b).[Part (b) courtesy of G.
Waller and P. Matsudaira.]Most eukaryotic cells contain all three types of cytoskeletal filaments, often concentrated in distinct locations. For example, in the absorptive epithelial cells thatline the lumen of the intestine, actin microfilaments areabundant in the apical region, where they are associatedwith cell–cell junctions and support a dense carpet of microvilli (Figure 5-30a). Actin filaments are also present ina narrow zone adjacent to the plasma membrane in the lateral regions of these cells. Keratin intermediate filaments,TABLE 5-4 Protein Subunits in Cytoskeletal FilamentsProtein SubunitsMWExpressionFunctionActin42,000Fungi, plant, animalStructural support, motilityMreB36,000Rod-shaped bacteriaWidth controlTubulin ( and )58,000Fungi, plant, animalStructural support, motility, cell polarityFtsZ58,000BacteriaCell divisionLaminsVariousPlant, animalSupport for nuclear membraneDesmin, keratin, vimentin, othersVariousAnimalCell adhesion50,000Nematode spermMotilityMICROFILAMENTSMICROTUBULESINTERMEDIATE FILAMENTSOTHERMSP5.4 • The Cytoskeleton: Components and Structural Functions(a)AFIFMT(b)175conservation is explained by the variety of critical functionsthat depend on the cytoskeleton.
A mutation in a cytoskeleton protein subunit could disrupt the assembly of filaments and their binding to other proteins. Analyses ofgene sequences and protein structures have identified bacterial homologs of actin and tubulin. The absence of IF-likeproteins in bacteria and unicellular eukaryotes is evidencethat intermediate filaments appeared later in the evolutionof the cytoskeletal system. The first IF protein to arise wasmost likely a nuclear lamin from which cytosolic IF proteins later evolved.The simple bacterial cytoskeleton controls cell length,width, and the site of cell division. The FtsZ protein, a bacterial homolog of tubulin, is localized around the neck of dividing bacterial cells, suggesting that FtsZ participates in celldivision (Figure 5-30b). The results of biochemical experiments with purified FtsZ demonstrate that it can polymerize into protofilaments, but these protofilaments do notassemble into intact microtubules.
Another bacterial protein,MreB, has been found to be similar to actin in atomic structure and filament structure—strong evidence that actinevolved from MreB. Clues to the function of MreB includeits localization in a filament that girdles rod-shaped bacterialcells, its absence from spherical bacteria, and the finding thatmutant cells lacking MreB become wider but not longer.These observations suggest MreB controls the width of rodshaped bacteria.(a)ActinMTsIFs(b)(c)MTAFIFCarbon filmforming a meshwork, connect microvilli and are tethered tojunctions between cells. Lamin intermediate filaments support the inner nuclear membrane.
Finally, microtubules,aligned with the long axis of the cell, are in close proximity to major cell organelles such as the endoplasmic reticulum, Golgi complex, and vesicles.The cytoskeleton has been highly conserved in evolution. A comparison of gene sequences shows only a smallpercentage of differences in sequence between yeast actinand tubulin and human actin and tubulin.
This structuralMreBFtsZMreB▲ FIGURE 5-30 Schematic depiction of the distribution ofcytoskeletal filaments in eukaryotic cells and bacterial cells.(a) In absorptive epithelial cells, actin filaments (red) areconcentrated in the apical region and in a narrow band in thebasolateral region. Microtubules (blue) are oriented with the longaxis of the cell, and intermediate filaments (green) areconcentrated along the cell periphery especially at specializedjunctions with neighboring cells and lining the nuclearmembrane.
(b) In a rod-shaped bacterial cell, filaments of MreB,the bacterial actin homolog, ring the cell and constrict its width.The bacterial tubulin homolog, FtsZ, forms filaments at the siteof cell division.176CHAPTER 5 • Biomembranes and Cell ArchitectureWe will consider various cytoskeletal cross-linking proteinsand their functions in Chapters 19 and 20.Cytoskeletal Filaments Are Organizedinto Bundles and NetworksOn first looking at micrographs of a cell, one is struck bythe dense, seemingly disorganized mat of filaments present inthe cytosol.
However, a keen eye will start to pick outareas—generally where the membrane protrudes from thecell surface or where a cell adheres to the surface or anothercell—in which the filaments are concentrated into bundles.From these bundles, the filaments continue into the cell interior, where they fan out and become part of a network offilaments. These two structures, bundles and networks, arethe most common arrangements of cytoskeletal filaments ina cell.Structurally, bundles differ from networks mainly in theorganization of the filaments. In bundles, the filaments areclosely packed in parallel arrays. In a network, the filaments crisscross, often at right angles, and are looselypacked. Networks can be further subdivided.
One type, associated with the nuclear and plasma membranes, is planar (two-dimensional), like a net or a web; the other type,present within the cell, is three-dimensional, giving thecytosol gel-like properties. In all bundles and networks, thefilaments are held together by various cross-linking proteins.(a)(b)Microfilaments and Membrane-BindingProteins Form a Skeleton Underlyingthe Plasma MembraneThe distinctive shape of a cell depends on the organization ofactin filaments and proteins that connect microfilaments to themembrane.
These proteins, called membrane–microfilamentbinding proteins, act as spot welds that tack the actincytoskeleton framework to the overlying membrane. Whenattached to a bundle of filaments, the membrane acquires thefingerlike shape of a microvillus or similar projection (see Figure 5-28). When attached to a planar network of filaments, themembrane is held flat like the red blood cell membrane. Thesimplest membrane–cytoskeleton connections entail the binding of integral membrane proteins directly to actin filaments.More common are complex linkages that connect actin filaments to integral membrane proteins through peripheral membrane proteins that function as adapter proteins. Such linkagesbetween the cytoskeleton and certain plasma-membrane proteins are considered in Chapter 6.Plasma membraneGlycophorinBand 3 dimerAnkyrinSpectrintetramerBand 4.1ActinTropomyosin0.1 µmBand 4.1AdducinTropomodulin▲ FIGURE 5-31 Cortical cytoskeleton supporting theplasma membrane in human erythrocytes.
(a) Electronmicrograph of the erythrocyte membrane showing the spokeand-hub organization of the cytoskeleton. The long spokes arecomposed mainly of spectrin and can be seen to intersect at thehubs, or membrane-attachment sites. The darker spots along thespokes are ankyrin molecules, which cross-link spectrin tointegral membrane proteins. (b) Diagram of the erythrocytecytoskeleton showing the various components.
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