What Does The Anther And Filament Makeup What Are The Levels Of Organization In The Human Body

The major cytoskeletal protein of well-nigh cells is actin, which polymerizes to form actin filaments—sparse, flexible fibers approximately 7 nm in diameter and up to several micrometers in length (Figure 11.1). Within the cell, actin filaments (also chosen microfilaments) are organized into higher-lodge structures, forming bundles or three-dimensional networks with the properties of semisolid gels. The assembly and disassembly of actin filaments, their crosslinking into bundles and networks, and their association with other cell structures (such every bit the plasma membrane) are regulated past a variety of actin-binding proteins, which are disquisitional components of the actin cytoskeleton. Actin filaments are particularly arable beneath the plasma membrane, where they course a network that provides mechanical support, determines cell shape, and allows motion of the jail cell surface, thereby enabling cells to migrate, engulf particles, and split.

Figure xi.i

Actin filaments. Electron micrograph of actin filaments. (Courtesy of Roger Craig, University of Massachusetts Medical Center.)

Assembly and Disassembly of Actin Filaments

Actin was first isolated from muscle cells, in which information technology constitutes approximately 20% of total prison cell protein, in 1942. Although actin was initially thought to be uniquely involved in musculus contraction, it is now known to be an extremely abundant poly peptide (typically 5 to 10% of total protein) in all types of eukaryotic cells. Yeasts accept merely a single actin cistron, but higher eukaryotes have several distinct types of actin, which are encoded by dissimilar members of the actin cistron family. Mammals, for instance, have at least six singled-out actin genes: Four are expressed in different types of musculus and ii are expressed in nonmuscle cells. All of the actins, still, are very similar in amino acrid sequence and take been highly conserved throughout the evolution of eukaryotes. Yeast actin, for case, is 90% identical in amino acid sequence to the actins of mammalian cells.

The three-dimensional structures of both individual actin molecules and actin filaments were adamant in 1990 by Kenneth Holmes, Wolfgang Kabsch, and their colleagues. Individual actin molecules are globular proteins of 375 amino acids (43 kd). Each actin monomer (globular [G] actin) has tight bounden sites that mediate head-to-tail interactions with two other actin monomers, so actin monomers polymerize to form filaments (filamentous [F] actin) (Effigy 11.2). Each monomer is rotated by 166^o in the filaments, which therefore take the appearance of a double-stranded helix. Because all the actin monomers are oriented in the same management, actin filaments have a distinct polarity and their ends (chosen the plus and minus ends) are distinguishable from one some other. This polarity of actin filaments is important both in their assembly and in establishing a unique direction of myosin movement relative to actin, as discussed later in the chapter.

Figure eleven.2

Associates and structure of actin filaments. (A) Actin monomers (G actin) polymerize to grade actin filaments (F actin). The kickoff step is the formation of dimers and trimers, which then abound by the addition of monomers to both ends. (B) Structure of an (more...)

The assembly of actin filaments can exist studied in vitro past regulation of the ionic strength of actin solutions. In solutions of low ionic strength, actin filaments depolymerize to monomers. Actin then polymerizes spontaneously if the ionic strength is increased to physiological levels. The first step in actin polymerization (chosen nucleation) is the germination of a small amass consisting of 3 actin monomers. Actin filaments are then able to grow by the reversible addition of monomers to both ends, only i end (the plus stop) elongates five to ten times faster than the minus end. The actin monomers also bind ATP, which is hydrolyzed to ADP following filament assembly. Although ATP is not required for polymerization, actin monomers to which ATP is bound polymerize more readily than those to which ADP is bound. As discussed below, ATP binding and hydrolysis play a key part in regulating the assembly and dynamic beliefs of actin filaments.

Because actin polymerization is reversible, filaments tin can depolymerize by the dissociation of actin subunits, allowing actin filaments to be broken down when necessary (Figure 11.iii). Thus, an apparent equilibrium exists between actin monomers and filaments, which is dependent on the concentration of gratis monomers. The rate at which actin monomers are incorporated into filaments is proportional to their concentration, so there is a disquisitional concentration of actin monomers at which the rate of their polymerization into filaments equals the charge per unit of dissociation. At this disquisitional concentration, monomers and filaments are in credible equilibrium.

Figure 11.3

Reversible polymerization of actin monomers. Actin polymerization is a reversible process, in which monomers both acquaintance with and dissociate from the ends of actin filaments. The rate of subunit dissociation (g _off) is independent of monomer concentration, (more...)

Equally noted earlier, the two ends of an actin filament grow at different rates, with monomers being added to the fast-growing end (the plus end) five to ten times faster than to the slow-growing (minus) end. Because ATP-actin dissociates less readily than ADP-actin, this results in a departure in the critical concentration of monomers needed for polymerization at the two ends. This difference tin can outcome in the miracle known as treadmilling, which illustrates the dynamic behavior of actin filaments (Figure 11.4). For the system to be at an overall steady state, the concentration of complimentary actin monomers must be intermediate betwixt the critical concentrations required for polymerization at the plus and minus ends of the actin filaments. Under these weather, there is a net loss of monomers from the minus end, which is counterbalanced by a net addition to the plus stop. Treadmilling requires ATP, with ATP-actin polymerizing at the plus end of filaments while ADP-actin dissociates from the minus end. Although the part of treadmilling in the prison cell is unclear, it may reflect the dynamic assembly and disassembly of actin filaments required for cells to move and change shape.

Figure eleven.four

Treadmilling. The minus ends grow less chop-chop than the plus ends of actin filaments. This difference in growth rate is reflected in a departure in the disquisitional concentration for add-on of monomers to the 2 ends of the filament. Actin bound to ATP (more...)

It is noteworthy that several drugs useful in cell biology deed by binding to actin and affecting its polymerization. For case, the cytochalasins bind to the plus ends of actin filaments and block their elongation. This results in changes in cell shape equally well as inhibition of some types of cell movements (due east.g., cell segmentation following mitosis), indicating that actin polymerization is required for these processes. Another drug, phalloidin, binds tightly to actin filaments and prevents their dissociation into private actin molecules. Phalloidin labeled with a fluorescent dye is often used to visualize actin filaments past fluorescence microscopy.

Inside the jail cell, both the assembly and disassembly of actin filaments are regulated past actin-bounden proteins (Figure 11.5). The turnover of actin filaments is about 100 times faster within the prison cell than it is in vitro, and this rapid turnover of actin plays a critical office in a variety of prison cell movements. The central protein responsible for actin filament disassembly within the cell is cofilin, which binds to actin filaments and enhances the rate of dissociation of actin monomers from the minus end. In add-on, cofilin can sever actin filaments, generating more ends and further enhancing filament disassembly.

Figure eleven.5

Effects of actin-binding proteins on filament turnover. Cofilin binds to actin filaments and increases the rate of dissociation of actin monomers (bound to ADP) from the minus finish. Cofilin remains spring to the ADP-actin monomers, preventing their reassembly (more...)

Cofilin preferentially binds to ADP-actin, so it remains leap to actin monomers following filament disassembly and sequesters them in the ADP-bound course, preventing their reincorporation into filaments. However, some other actin-bounden protein, profilin, can reverse this effect of cofilin and stimulate the incorporation of actin monomers into filaments. Profilin acts by stimulating the exchange of jump ADP for ATP, resulting in the formation of ATP-actin monomers, which dissociate from cofilin and are then available for assembly into filaments. Other proteins (Arp2/3 proteins) can serve as nucleation sites to initiate the assembly of new filaments, so cofilin, profilin, and the Arp2/iii proteins (too as other actin-bounden proteins) can deed together to promote the rapid turnover of actin filaments and remodeling of the actin cytoskeleton which is required for a variety of cell movements and changes in cell shape. Every bit might be expected, the activities of cofilin, profilin, and Arp2/3 proteins are controlled by a multifariousness of cell signaling mechanisms (discussed in Chapter thirteen), allowing actin polymerization to be appropriately regulated in response to environmental stimuli.

Organization of Actin Filaments

Individual actin filaments are assembled into two general types of structures, chosen actin bundles and actin networks, which play dissimilar roles in the prison cell (Figure xi.6). In bundles, the actin filaments are crosslinked into closely packed parallel arrays. In networks, the actin filaments are loosely crosslinked in orthogonal arrays that form three-dimensional meshworks with the properties of semisolid gels. The germination of these structures is governed by a variety of actin-binding proteins that crosslink actin filaments in singled-out patterns.

Figure xi.vi

Actin bundles and networks. (A) Electron micrograph of actin bundles (arrowheads) projecting from the actin network (arrows) underlying the plasma membrane of a macrophage. The bundles support cell surface projections called microspikes or filopodia (meet (more than...)

All of the actin-bounden proteins involved in crosslinking contain at to the lowest degree two domains that bind actin, allowing them to bind and crosslink ii different actin filaments. The nature of the association between these filaments is then adamant by the size and shape of the crosslinking proteins (see Figure 11.6). The proteins that crosslink actin filaments into bundles (called actin-bundling proteins) usually are small-scale rigid proteins that force the filaments to align closely with ane another. In contrast, the proteins that organize actin filaments into networks tend to exist large flexible proteins that can crosslink perpendicular filaments. These actin-crosslinking proteins appear to be modular proteins consisting of related structural units. In item, the actin-bounden domains of many of these proteins are like in structure. They are separated past spacer sequences that vary in length and flexibility, and information technology is these differences in the spacer sequences that are responsible for the singled-out crosslinking properties of unlike actin-binding proteins.

There are two structurally and functionally singled-out types of actin bundles, involving different actin-bundling proteins (Effigy 11.7). The first type of bundle, containing closely spaced actin filaments aligned in parallel, supports projections of the plasma membrane, such as microvilli (see Figures 11.15 and 11.sixteen). In these bundles, all the filaments have the same polarity, with their plus ends next to the plasma membrane. An example of a bundling protein involved in the formation of these structures is fimbrin, which was kickoff isolated from intestinal microvilli and subsequently found in surface projections of a wide multifariousness of cell types. Fimbrin is a 68-kd protein, containing 2 side by side actin-binding domains. It binds to actin filaments every bit a monomer, holding two parallel filaments close together.

Effigy 11.7

Actin-bundling proteins. Actin filaments are associated into ii types of bundles past different actin-bundling proteins. Fimbrin has two side by side actin-binding domains (ABD) and crosslinks actin filaments into closely packed parallel bundles in which the (more...)

Figure 11.15

Electron micrograph of microvilli. The microvilli (arrows) of intestinal epithelial cells are fingerlike projections of the plasma membrane. They are supported by actin bundles anchored in a dumbo region of the cortex called the last spider web. (Fred E. (more...)

Figure 11.16

Organisation of microvilli. The core actin filaments of microvilli are crosslinked into closely packed bundles past fimbrin and villin. They are attached to the plasma membrane forth their length by lateral arms, consisting of myosin I and calmodulin. The (more...)

The 2d type of actin bundle is equanimous of filaments that are more loosely spaced and are capable of contraction, such as the actin bundles of the contractile band that divides cells in two following mitosis. The looser structure of these bundles (which are called contractile bundles) reflects the backdrop of the crosslinking protein α-actinin. In contrast to fimbrin, α-actinin binds to actin as a dimer, each subunit of which is a 102-kd poly peptide containing a single actin-binding site. Filaments crosslinked by α-actinin are consequently separated by a greater distance than those crosslinked past fimbrin (forty nm apart instead of 14 nm). The increased spacing betwixt filaments allows the motor protein myosin to interact with the actin filaments in these bundles, which (as discussed later) enables them to contract.

The actin filaments in networks are held together by large actin-binding proteins, such as filamin (Figure 11.eight). Filamin (as well called actin-binding protein or ABP-280) binds actin as a dimer of two 280-kd subunits. The actin-binding domains and dimerization domains are at opposite ends of each subunit, so the filamin dimer is a flexible V-shaped molecule with actin-binding domains at the ends of each arm. Every bit a upshot, filamin forms cross-links between orthogonal actin filaments, creating a loose three-dimensional meshwork. As discussed in the next department, such networks of actin filaments underlie the plasma membrane and support the surface of the jail cell.

Effigy 11.viii

Actin networks and filamin. Filamin is a dimer of 2 big (280-kd) subunits, forming a flexible V-shaped molecule that crosslinks actin filaments into orthogonal networks. The carboxy-terminal dimerization domain is separated from the amino-terminal (more...)

Association of Actin Filaments with the Plasma Membrane

Actin filaments are highly concentrated at the periphery of the prison cell, where they grade a three-dimensional network beneath the plasma membrane (see Effigy 11.6). This network of actin filaments and associated actin-binding proteins (called the cell cortex) determines cell shape and is involved in a variety of prison cell surface activities, including motility. The association of the actin cytoskeleton with the plasma membrane is thus central to cell construction and part.

Red claret cells (erythrocytes) have proven especially useful for studies of both the plasma membrane (discussed in the next chapter) and the cortical cytoskeleton. The principal advantage of ruby blood cells for these studies is that they contain no nucleus or internal organelles, so their plasma membrane and associated proteins can be easily isolated without contagion by the diverse internal membranes that are abundant in other cell types. In add-on, human being erythrocytes lack other cytoskeletal components (microtubules and intermediate filaments), so the cortical cytoskeleton is the principal determinant of their distinctive shape as biconcave discs (Figure 11.9).

Figure 11.9

Morphology of ruddy blood cells. Scanning electron micrograph of red blood cells illustrating their biconcave shape. (Omikron/Photo Researchers, Inc.)

The major poly peptide that provides the structural basis for the cortical cytoskeleton in erythrocytes is the actin-binding poly peptide spectrin, which is related to filamin (Effigy 11.10). Erythrocyte spectrin is a tetramer consisting of ii distinct polypeptide bondage, chosen α and β, with molecular weights of 240 and 220 kd, respectively. The β chain has a single actin-binding domain at its amino terminus. The α and β chains associate laterally to course dimers, which then join head to head to form tetramers with 2 actin-binding domains separated past approximately 200 nm. The ends of the spectrin tetramers then acquaintance with brusque actin filaments, resulting in the spectrin-actin network that forms the cortical cytoskeleton of cerise claret cells (Figure 11.11). The major link between the spectrin-actin network and the plasma membrane is provided by a poly peptide called ankyrin, which binds both to spectrin and to the cytoplasmic domain of an abundant transmembrane protein called band 3. An additional link between the spectrin-actin network and the plasma membrane is provided past poly peptide four.ane, which binds to spectrin-actin junctions too as recognizing the cytoplasmic domain of glycophorin (another abundant transmembrane protein).

Effigy 11.10

Structure of spectrin. Spectrin is a tetramer consisting of two α and two β bondage. Each β chain has a single actin-binding domain (ABD) at its amino terminus. Both α and β bondage contain multiple repeats of α-helical (more...)

Figure xi.11

Association of the erythrocyte cortical cytoskeleton with the plasma membrane. The plasma membrane is associated with a network of spectrin tetramers crosslinked past brusk actin filaments in clan with protein four.1. The spectrin-actin network is linked (more...)

Other types of cells contain linkages between the cortical cytoskeleton and the plasma membrane that are similar to those observed in red blood cells. Proteins related to spectrin (nonerythroid spectrin is also called fodrin), ankyrin, and poly peptide 4.1 are expressed in a broad range of cell types, where they fulfill functions analogous to those described for erythrocytes. For example, a family of proteins related to poly peptide iv.ane (the ERM proteins) link actin filaments to the plasma membranes of many different kinds of cells and the spectrin-related poly peptide filamin (see Figure 11.8) constitutes a major link betwixt actin filaments and the plasma membrane of blood platelets. Another member of this grouping of spectrin-related proteins is dystrophin, which is of particular interest because it is the product of the gene responsible for 2 types of muscular dystrophy (Duchenne'south and Becker'south). These X-linked inherited diseases event in progressive degeneration of skeletal muscle, and patients with the more severe form of the affliction (Duchenne's muscular dystrophy) usually die in their teens or early twenties. Molecular cloning of the gene responsible for this disorder revealed that it encodes a large protein (427 kd) that is either absent or abnormal in patients with Duchenne'due south or Becker's muscular dystrophy, respectively. The sequence of dystrophin further indicated that it is related to spectrin, with a single actin-binding domain at its amino terminus and a membrane-bounden domain at its carboxy terminus. Similar spectrin, dystrophin forms dimers that link actin filaments to transmembrane proteins of the musculus cell plasma membrane. These transmembrane proteins in turn link the cytoskeleton to the extracellular matrix, which plays an important part in maintaining cell stability during muscle contraction.

In dissimilarity to the compatible surface of carmine blood cells, about cells have specialized regions of the plasma membrane that form contacts with next cells, tissue components, or other substrates (such equally the surface of a culture dish). These regions as well serve as attachment sites for bundles of actin filaments that anchor the cytoskeleton to areas of cell contact. These attachments of actin filaments are particularly evident in fibroblasts maintained in tissue civilisation (Effigy 11.12). Such cultured fibroblasts secrete extracellular matrix proteins (discussed in Chapter 12) that stick to the plastic surface of the civilisation dish. The fibroblasts so adhere to the culture dish via the binding of transmembrane proteins (chosen integrins) to the extracellular matrix. The sites of zipper are discrete regions (called focal adhesions) that too serve equally attachment sites for big bundles of actin filaments chosen stress fibers.

Figure eleven.12

Stress fibers and focal adhesions. Fluorescence microscopy of a human being fibroblast in which actin filaments accept been been stained with a fluorescent dye. Stress fibers are revealed every bit bundles of actin filaments anchored at sites of jail cell attachment to the (more...)

Stress fibers are contractile bundles of actin filaments, crosslinked by α-actinin, that anchor the cell and exert tension against the substratum. They are fastened to the plasma membrane at focal adhesions via interactions with integrin. These associations, which are complex and not well understood, may be mediated by several other proteins, including talin and vinculin (Figure xi.thirteen). For example, both talin and α-actinin bind to the cytoplasmic domains of integrins. Talin too binds to vinculin, which in plough interacts with actin. Other proteins found at focal adhesions may too participate in the attachment of actin filaments, and a combination of these interactions may be responsible for the linkage of actin filaments to the plasma membrane.

Figure eleven.thirteen

Attachment of stress fibers to the plasma membrane at focal adhesions. Focal adhesions are mediated past the binding of integrins to proteins of the extracellular matrix. Stress fibers (bundles of actin filaments crosslinked by α-actinin) are so (more...)

The actin cytoskeleton is similarly anchored to regions of cell-cell contact called adherens junctions (Figure xi.14). In sheets of epithelial cells, these junctions grade a continuous beltlike structure (chosen an adhesion belt) around each jail cell in which an underlying contractile package of actin filaments is linked to the plasma membrane. Contact between cells at adherens junctions is mediated by transmembrane proteins called cadherins, which are discussed farther in Chapter 12. The cadherins form a complex with cytoplasmic proteins called catenins, which associate with actin filaments.

Effigy xi.14

Attachment of actin filaments to adherens junctions. Cell-cell contacts at adherens junctions are mediated by cadherins, which serve equally sites of attachment of actin bundles. In sheets of epithelial cells, these junctions form a continuous belt of actin (more...)

Protrusions of the Prison cell Surface

The surfaces of most cells have a diverseness of protrusions or extensions that are involved in cell movement, phagocytosis, or specialized functions such every bit absorption of nutrients. Most of these jail cell surface extensions are based on actin filaments, which are organized into either relatively permanent or rapidly rearranging bundles or networks.

The best-characterized of these actin-based prison cell surface protrusions are microvilli, fingerlike extensions of the plasma membrane that are particularly arable on the surfaces of cells involved in absorption, such as the epithelial cells lining the intestine (Figure 11.15). The microvilli of these cells class a layer on the apical surface (called a castor border) that consists of approximately a yard microvilli per prison cell and increases the exposed surface area available for absorption past 10- to xx-fold. In addition to their part in absorption, specialized forms of microvilli, the stereocilia of auditory hair cells, are responsible for hearing past detecting sound vibrations.

Their abundance and ease of isolation have facilitated detailed structural analysis of abdominal microvilli, which comprise closely packed parallel bundles of 20 to 30 actin filaments (Figure 11.16). The filaments in these bundles are crosslinked in part past fimbrin, an actin-bundling protein (discussed earlier) that is present in surface projections of a variety of cell types. However, the major actin-bundling protein in abdominal microvilli is villin, a 95-kd poly peptide present in microvilli of only a few specialized types of cells, such as those lining the intestine and kidney tubules. Along their length, the actin bundles of microvilli are attached to the plasma membrane by lateral arms consisting of the calcium-binding protein calmodulin in association with myosin I, which may exist involved in movement of the plasma membrane along the actin packet of the microvillus. At their base, the actin bundles are anchored in a spectrin-rich region of the actin cortex chosen the last web, which crosslinks and stabilizes the microvilli.

In contrast to microvilli, many surface protrusions are transient structures that form in response to environmental stimuli. Several types of these structures extend from the leading edge of a moving cell and are involved in cell locomotion (Effigy 11.17). Pseudopodia are extensions of moderate width, based on actin filaments crosslinked into a iii-dimensional network, that are responsible for phagocytosis and for the motion of amoebas across a surface. Lamellipodia are wide, sheetlike extensions at the leading edge of fibroblasts, which similarly contain a network of actin filaments. Many cells also extend microspikes or filopodia, sparse projections of the plasma membrane supported by actin bundles. The germination and retraction of these structures is based on the regulated associates and disassembly of actin filaments, as discussed in the following section.

Figure 11.17

Examples of cell surface projections involved in phagocytosis and movement. (A) Scanning electron micrograph showing pseudopodia of a macrophage engulfing a tumor prison cell during phagocytosis. (B) An amoeba with several extended pseudopodia. (C) A tissue (more...)

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Molecular Medicine: Muscular Dystrophy and the Cytoskeleton.

Source: https://www.ncbi.nlm.nih.gov/books/NBK9908/

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