Lodish H. - Molecular Cell Biology (5ed, Freeman, 2003) (794361), страница 93
Текст из файла (страница 93)
Mutual repulsion between negatively chargedcarboxylate groups that protrude outward at regular intervals also contributes to these local rigid structures. Overall,however, hyaluronan is not a long, rigid rod as is fibrillar collagen; rather, in solution it is very flexible, bending and twisting into many conformations, forming a random coil.Because of the large number of anionic residues on itssurface, the typical hyaluronan molecule binds a largeamount of water and behaves as if it were a large hydratedsphere with a diameter of ≈500 nm.
As the concentration ofhyaluronan increases, the long chains begin to entangle,forming a viscous gel. Even at low concentrations, hyaluronan forms a hydrated gel; when placed in a confining space,such as in a matrix between two cells, the long hyaluronanmolecules will tend to push outward. This outward pushingcreates a swelling, or turgor pressure, within the extracellular space.
In addition, the binding of cations by COOgroups on the surface of hyaluronan increases the concentration of ions and thus the osmotic pressure in the gel. As aresult, large amounts of water are taken up into the matrix,contributing to the turgor pressure. These swelling forcesgive connective tissues their ability to resist compressionforces, in contrast with collagen fibers, which are able to resist stretching forces.Hyaluronan is bound to the surface of many migratingcells by a number of adhesion receptors (e.g., one calledCD44) containing HA-binding domains, each with a similarthree-dimensional conformation. Because of its loose, hydrated, porous nature, the hyaluronan “coat” bound to cellsappears to keep cells apart from one another, giving themthe freedom to move about and proliferate.
The cessation ofcell movement and the initiation of cell–cell attachments arefrequently correlated with a decrease in hyaluronan, a decrease in HA-binding cell-surface molecules, and an increasein the extracellular enzyme hyaluronidase, which degradeshyaluronan in the matrix. These functions of hyaluronanare particularly important during the many cell migrationsthat facilitate differentiation and in the release of amammalian egg cell (oocyte) from its surrounding cells afterovulation.Association of Hyaluronan and ProteoglycansForms Large, Complex AggregatesThe predominant proteoglycan in cartilage, called aggrecan,assembles with hyaluronan into very large aggregates, illustrative of the complex structures that proteoglycans sometimes form. The backbone of the cartilage proteoglycan220CHAPTER 6 • Integrating Cells into Tissuesaggregate is a long molecule of hyaluronan to which multipleaggrecan molecules are bound tightly but noncovalently (Figure 6-22a).
A single aggrecan aggregate, one of the largestmacromolecular complexes known, can be more than 4 mmlong and have a volume larger than that of a bacterial cell.Hyaluronan molecule(a)Aggrecan(b)300 nmHyaluronan moleculeLink proteinKeratansulfateN-terminalHyaluronan-bindingdomainChondroitinsulfateLinkingsugarsAggrecan core protein▲ FIGURE 6-22 Structure of proteoglycan aggregate fromcartilage. (a) Electron micrograph of an aggrecan aggregate fromfetal bovine epiphyseal cartilage. Aggrecan core proteins arebound at ≈40-nm intervals to a molecule of hyaluronan.(b) Schematic representation of an aggrecan monomer bound tohyaluronan.
In aggrecan, both keratan sulfate and chondroitinsulfate chains are attached to the core protein. The N-terminaldomain of the core protein binds noncovalently to a hyaluronanmolecule. Binding is facilitated by a link protein, which binds toboth the hyaluronan molecule and the aggrecan core protein.Each aggrecan core protein has 127 Ser-Gly sequences at whichGAG chains can be added. The molecular weight of an aggrecanmonomer averages 2 106. The entire aggregate, which maycontain upward of 100 aggrecan monomers, has a molecularweight in excess of 2 108. [Part (a) from J. A.
Buckwalter andL. Rosenberg, 1983, Coll. Rel. Res. 3:489; courtesy of L. Rosenberg.]These aggregates give cartilage its unique gel-like propertiesand its resistance to deformation, essential for distributingthe load in weight-bearing joints.The aggrecan core protein (≈250,000 MW) has one Nterminal globular domain that binds with high affinity to aspecific decasaccharide sequence within hyaluronan. Thisspecific sequence is generated by covalent modification ofsome of the repeating disaccharides in the hyaluronan chain.The interaction between aggrecan and hyaluronan is facilitated by a link protein that binds to both the aggrecan coreprotein and hyaluronan (Figure 6-22b). Aggrecan and thelink protein have in common a “link” domain, ≈100 aminoacids long, that is found in numerous matrix and cellsurface hyaluronan-binding proteins in both cartilaginousand noncartilaginous tissues.
Almost certainly these proteinsarose in the course of evolution from a single ancestral genethat encoded just this domain.The importance of the GAG chains that are part ofvarious matrix proteoglycans is illustrated by therare humans who have a genetic defect in one ofthe enzymes required for synthesis of the GAG dermatan sulfate. These persons have many defects in their bones, joints,and muscles; do not grow to normal height; and have wrinkled skin, giving them a prematurely aged appearance. ❚Fibronectins Connect Many Cells to FibrousCollagens and Other Matrix ComponentsMany different cell types synthesize fibronectin, an abundantmultiadhesive matrix protein found in all vertebrates.
Thediscovery that fibronectin functions as an adhesive moleculestemmed from observations that it is present on the surfacesof normal fibroblastic cells, which adhere tightly to petridishes in laboratory experiments, but is absent from the surfaces of tumorigenic cells, which adhere weakly. The 20 or soisoforms of fibronectin are generated by alternative splicingof the RNA transcript produced from a single gene (see Figure 4-15). Fibronectins are essential for the migration anddifferentiation of many cell types in embryogenesis. Theseproteins are also important for wound healing because theypromote blood clotting and facilitate the migration ofmacrophages and other immune cells into the affected area.Fibronectins help attach cells to the extracellular matrixby binding to other ECM components, particularly fibrouscollagens and heparan sulfate proteoglycans, and to cellsurface adhesion receptors such as integrins (see Figure 6-2).Through their interactions with adhesion receptors (e.g.,51 integrin), fibronectins influence the shape and movement of cells and the organization of the cytoskeleton.
Conversely, by regulating their receptor-mediated attachmentsto fibronectin and other ECM components, cells can sculptthe immediate ECM environment to suit their needs.Fibronectins are dimers of two similar polypeptideslinked at their C-termini by two disulfide bonds; each chainis about 60–70 nm long and 2–3 nm thick. Partial digestion2216.4 • The Extracellular Matrix of Nonepithelial TissuesFibrin,heparansulfate–bindingrepeatsCollagenbindingrepeatsHeparansulfate–bindingrepeatIntegrinbindingrepeatsFibrinbindingrepeatsCOOHNH2EIIIBRepeating amino acid sequences:Type IType IIEIIIAIIICSType III▲ FIGURE 6-23 Organization of fibronectin chains.
Only oneof the two chains present in the dimeric fibronectin molecule isshown; both chains have very similar sequences. Each chaincontains about 2446 amino acids and is composed of three typesof repeating amino acid sequences. Circulating fibronectin lacksone or both of the type III repeats designated EIIIA and EIIIBowing to alternative mRNA splicing (see Figure 4-15). At leastfive different sequences may be present in the IIICS region as aof fibronectin with low amounts of proteases and analysisof the fragments showed that each chain comprises six functional regions with different ligand-binding specificities (Figure 6-23).
Each region, in turn, contains multiple copies ofcertain sequences that can be classified into one of threetypes. These classifications are designated fibronectin type I,II, and III repeats, on the basis of similarities in amino acidsequence, although the sequences of any two repeats of agiven type are not always identical. These linked repeats givethe molecule the appearance of beads on a string. The combination of different repeats composing the regions, anotherexample of combinatorial diversity, confers on fibronectin itsability to bind multiple ligands.S SS SCOOHresult of alternative splicing.
Each chain contains six domains (tanboxes), some of which contain specific binding sites for heparansulfate, fibrin (a major constituent of blood clots), collagen, andcell-surface integrins. The integrin-binding domain is also knownas the cell-binding domain. [Adapted from G. Paolella, M. Barone,and F. Baralle, 1993, in M. Zern and L. Reid, eds., Extracellular Matrix,Marcel Dekker, pp. 3–24.]One of the type III repeats in the cell-binding region of fibronectin mediates binding to certain integrins.
The resultsof studies with synthetic peptides corresponding to parts ofthis repeat identified the tripeptide sequence Arg-Gly-Asp,usually called the RGD sequence, as the minimal sequencewithin this repeat required for recognition by those integrins.In one study, heptapeptides containing the RGD sequence ora variation of this sequence were tested for their ability tomediate the adhesion of rat kidney cells to a culture dish. Theresults showed that heptapeptides containing the RGDsequence mimicked intact fibronectin’s ability to stimulateintegrin-mediated adhesion, whereas variant heptapeptideslacking this sequence were ineffective (Figure 6-24).Relative amounts of bound cells (stain intensity)1.4 EXPERIMENTAL FIGURE 6-24 A specific tripeptideGRGDSPC1.2GRGDAPCPRGDVDC1.0YKPGEGKRGDACEGDSG0.80.60.4GRADSPCGRGESPCGKGDSPCDREDSRC0.21101001000Peptide concentration (nmol/ml)sequence (RGD) in the cell-binding region of fibronectin isrequired for adhesion of cells.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
