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Cooper GM. The Cell: A molecule Approach. Second edition. Sunderland (MA): Sinauer Associates; 2000.


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Actin filaments, usually in association with myosin, room responsible for many types of cell movements. Myosin is the prototype the a molecular motor—a protein that converts chemical power in the form of ATP to mechanical energy, thus generating force and also movement. The many striking range of such movement is muscle contraction, which has provided the design for understanding actin-myosin interactions and also the motor task of myosin molecules. However, interactions of actin and myosin room responsible not just for muscle convulsion but also for a selection of motions of nonmuscle cells, including cell division, so these interactions play a main role in cell biology. Moreover, the actin cytoskeleton is responsible because that the crawling movements of cells throughout a surface, which appear to it is in driven straight by actin polymerization and actin-myosin interactions.


Muscle Contraction

Muscle cells room highly dedicated for a solitary task, contraction, and also it is this field of expertise in structure and role that has made muscle the prototype because that studying movement at the cellular and molecular levels. There are three distinct species of muscle cells in vertebrates: skeleton muscle, i beg your pardon is responsible for all voluntary movements; cardiac muscle, i beg your pardon pumps blood from the heart; and smooth muscle, i beg your pardon is responsible because that involuntary activities of offal such as the stomach, intestine, uterus, and also blood vessels. In both skeletal and cardiac muscle, the contractile elements of the cytoskeleton are existing in very organized arrays that offer rise come characteristic trends of cross-striations. That is the characterization the these structures in skeletal muscle that has led to our existing understanding of muscle contraction, and also other actin-based cell movements, at the molecular level.

Skeletal muscles space bundles that muscle fibers, which space single huge cells (approximately 50 μm in diameter and also up to several centimeters in length) developed by the blend of numerous individual cell during advancement (Figure 11.18). Many of the cytoplasm consists of myofibrils, which room cylindrical bundles of two varieties of filaments: special filaments that myosin (about 15 nm in diameter) and also thin filaments the actin (about 7 nm in diameter). Each myofibril is arranged as a chain of contractile units called sarcomeres, which room responsible for the striated appearance of skeletal and also cardiac muscle.


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Figure 11.18

Structure of muscle cells. Muscles are composed of bundles of single large cells (called muscle fibers) that form by cell blend and save on computer multiple nuclei. Every muscle fiber has many myofibrils, which space bundles that actin and myosin filaments arranged (more...)


The sarcomeres (which are roughly 2.3 μm long) consist of several distinct regions, discernible by electron microscopy, which provided crucial insights right into the device of muscle contraction (Figure 11.19). The end of each sarcomere are characterized by the Z disc. Within each sarcomere, dark bands (called A bands since they space anisotropic when viewed v polarized light) alternating with light bands (called i bands for isotropic). These bands correspond to the visibility or absence of myosin filaments. The ns bands contain just thin (actin) filaments, conversely, the A bands contain special (myosin) filaments. The myosin and actin filaments overlap in peripheral areas of the A band, whereas a middle region (called the H zone) includes only myosin. The actin filaments are attached at your plus ends to the Z disc, which consists of the crosslinking protein α-actinin. The myosin filaments are anchored in ~ the M line in the center of the sarcomere.


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Figure 11.19

Structure that the sarcomere. (A) Electron micrograph the a sarcomere. (B) Diagram showing the organization of actin (thin) and also myosin (thick) filaments in the indicated regions. (A, frank A. Pepe/Biological photo Service.)


Two additional proteins (titin and also nebulin) likewise contribute come sarcomere structure and stability (Figure 11.20). Titin is one extremely large protein (3000 kd), and single titin molecules expand from the M line to the Z disc. These lengthy molecules that titin room thought come act favor springs that save the myosin filaments focused in the sarcomere and maintain the relaxing tension that allows a muscle come snap back if overextended. Nebulin filaments are connected with actin and are thought to control the assembly the actin filaments by acting together rulers that recognize their length.


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Figure 11.20

Titin and also nebulin. Molecule of titin extend from the Z disc come the M line and also act as springs to save myosin filaments centered in the sarcomere. Molecules of nebulin extend from the Z disc and also are believed to identify the length of connected actin filaments. (more...)


The basis for expertise muscle convulsion is the sliding filament model, an initial proposed in 1954 both by Andrew Huxley and Ralph Niedergerke and also by Hugh Huxley and Jean Hanson (Figure 11.21). During muscle contraction, every sarcomere shortens, pass the Z discs closer together. There is no change in the broad of the A band, but both the ns bands and also the H region almost fully disappear. These transforms are explained by the actin and also myosin filaments sliding previous one another, so the the actin filaments move into the A band and also H zone. Muscle contraction therefore results native an interaction between the actin and myosin filaments the generates their motion relative come one another. The molecular basis because that this interaction is the binding the myosin to actin filaments, enabling myosin to function as a motor the drives filament sliding.


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Figure 11.21

Sliding-filament version of muscle contraction. The actin filaments slide previous the myosin filaments toward the middle of the sarcomere. The result is shortening the the sarcomere there is no any readjust in filament length.


The form of myosin current in muscle (myosin II) is a very huge protein (about 500 kd) consisting of 2 identical heavy chains (about 200 kd each) and two pairs of light chains (about 20 kd each) (Figure 11.22). Each hefty chain is composed of a globular head an ar and a long α-helical tail. The α-helical tails that two heavy chains twist roughly each other in a coiled-coil structure to form a dimer, and two irradiate chains associate v the neck of each head region to kind the complete myosin II molecule.


Figure 11.22

Myosin II. The myosin II molecule is composed of two heavy chains and two pairs of irradiate chains (called the essential and regulatory irradiate chains). The hefty chains have globular head regions and also long α-helical tails, i beg your pardon coil approximately each other (more...)


The special filaments that muscle consists of several hundred myosin molecules, linked in a parallel staggered selection by interactions in between their tails (Figure 11.23). The globular top of myosin tie actin, forming cross-bridges between the thick and also thin filaments. That is essential to keep in mind that the orientation of myosin molecules in the thick filaments reverses at the M heat of the sarcomere. The polarity the actin filaments (which room attached come Z discs at their plus ends) likewise reverses in ~ the M line, therefore the family member orientation of myosin and actin filaments is the very same on both halves the the sarcomere. As questioned later, the motor activity of myosin move its head teams along the actin filament in the direction of the add to end. This motion slides the actin filaments from both sides of the sarcomere towards the M line, shortening the sarcomere and also resulting in muscle contraction.


Figure 11.23

Organization the myosin thick filaments. Special filaments are formed by the association of several hundred myosin II molecules in a staggered array. The globular top of myosin tie actin, forming cross-bridges between the myosin and also actin filaments. The (more...)


In enhancement to binding actin, the myosin heads bind and hydrolyze ATP, which provides the energy to journey filament sliding. This translate in of chemical power to movement is mediated by transforms in the shape of myosin result from ATP binding. The generally accepted model (the swinging-cross-bridge model) is that ATP hydrolysis drives repeated cycles that interaction between myosin heads and also actin. During each cycle, conformational changes in myosin an outcome in the motion of myosin heads along actin filaments.

Although the molecule mechanisms space still not completely understood, a plausible working model for myosin function has been derived both from in vitro studies of myosin movement along actin filaments (a system developed by James Spudich and also Michael Sheetz) and from determination of the three-dimensional structure of myosin by Ivan Rayment and his colleagues (Figure 11.24). The bike starts with myosin (in the absence of ATP) strict bound come actin. ATP binding dissociates the myosin-actin complicated and the hydrolysis that ATP climate induces a conformational change in myosin. This change affects the neck region of myosin that binds the irradiate chains (see number 11.22), which acts as a bar arm to displace the myosin head by around 5 nm. The products of hydrolysis (ADP and Pi) remain bound to the myosin head, which is claimed to it is in in the “cocked” position. The myosin head then rebinds at a new position top top the actin filament, leading to the release of ADP and also Pi and triggering the “power stroke,” in which the myosin head returns to its early stage conformation, thereby sliding the actin filament towards the M heat of the sarcomere.


Figure 11.24

Model for myosin action. The binding of ATP dissociates myosin native actin. ATP hydrolysis then induces a conformational change that displaces the myosin head group. This is complied with by binding the the myosin head to a brand-new position ~ above the actin filament (more...)


The convulsion of skeletal muscle is triggered by nerve impulses, which stimulate the relax of Ca2+ from the sarcoplasmic reticulum—a specialized network of interior membranes, comparable to the absorbent reticulum, the stores high concentration of Ca2+ ions. The relax of Ca2+ indigenous the sarcoplasmic reticulum rises the concentration that Ca2+ in the cytosol from around 10-7 come 10-5M. The raised Ca2+ concentration signals muscle contraction via the activity of 2 accessory protein bound to the actin filaments: tropomyosin and troponin (Figure 11.25). Tropomyosin is a fibrous protein that binds lengthwise along the groove of actin filaments. In striated muscle, each tropomyosin molecule is bound come troponin, i beg your pardon is a complicated of three polypeptides: troponin C (Ca2+-binding), troponin i (inhibitory), and also troponin T (tropomyosin-binding). When the concentration of Ca2+ is low, the complicated of the troponins with tropomyosin block the interaction of actin and myosin, so the muscle does not contract. At high concentrations, Ca2+ binding to troponin C shifts the place of the complex, relieving this inhibition and enabling contraction come proceed.


Figure 11.25

Association that tropomyosin and also troponins through actin filaments. (A) Tropomyosin binds lengthwise along actin filaments and, in striated muscle, is linked with a complex of 3 troponins: troponin i (TnI), troponin C (TnC), and troponin T (TnT). In (more...)


Contractile Assemblies that Actin and also Myosin in Nonmuscle Cells

Contractile assemblies that actin and myosin, the same, similar thing small-scale versions of muscle fibers, are present additionally in nonmuscle cells. Together in muscle, the actin filaments in this contractile assemblies room interdigitated through bipolar filaments the myosin II, consists of 15 come 20 myosin II molecules, which produce contraction by sliding the actin filaments family member to one another (Figure 11.26). The actin filaments in contractile bundles in nonmuscle cell are likewise associated through tropomyosin, i m sorry facilitates their communication with myosin II, most likely by contending with filamin for binding website on actin.


Figure 11.26

Contractile assemblies in nonmuscle cells. Bipolar filaments the myosin II develop contraction by slide actin filaments in the contrary directions.


Two instances of contractile assemblies in nonmuscle cells, tension fibers and adhesion belts, were disputed earlier through respect to attachments of the actin cytoskeleton to regions of cell-substrate and also cell-cell contacts (see figures 11.13 and 11.14). The contraction of anxiety fibers to produce tension throughout the cell, allowing the cabinet to pull on a substrate (e.g., the extracellular matrix) come which the is anchored. The contraction of adhesion belts changes the shape of epithelial cabinet sheets: a procedure that is an especially important throughout embryonic development, once sheets that epithelial cells fold right into structures such together tubes.

The most dramatic instance of actin-myosin convulsion in nonmuscle cells, however, is provided by cytokinesis—the department of a cell right into two following mitosis (Figure 11.27). Toward the finish of mitosis in pet cells, a contractile ring consist of of actin filaments and myosin II assembles just underneath the plasma membrane. That contraction pulls the plasma membrane progressively inward, constricting the center of the cell and pinching the in two. Interestingly, the thickness that the contractile ring remains consistent as the contracts, implying that actin filaments disassemble as contraction proceeds. The ring climate disperses totally following cabinet division.


Figure 11.27

Cytokinesis. Following completion the mitosis (nuclear division), a contractile ring consisting of actin filaments and also myosin II divides the cabinet in two.


The regulation of actin-myosin convulsion in striated muscle, questioned earlier, is mediated by the binding of Ca2+ to troponin. In nonmuscle cells and in smooth muscle, however, contraction is regulated primarily by phosphorylation of one of the myosin light chains, referred to as the regulatory light chain (Figure 11.28). Phosphorylation the the regulatory light chain in this cells contends least 2 effects: It promotes the assembly of myosin right into filaments, and it boosts myosin catalytic activity, permitting contraction to proceed. The enzyme that catalyzes this phosphorylation, called myosin light-chain kinase, is chin regulated by association with the Ca2+-binding protein calmodulin. Rises in cytosolic Ca2+ promote the binding of calmodulin to the kinase, leading to phosphorylation the the myosin regulatory light chain. Increases in cytosolic Ca2+ are for this reason responsible, albeit indirectly, because that activating myosin in smooth muscle and nonmuscle cells, and also in striated muscle.


Figure 11.28

Regulation the myosin through phosphorylation. Ca2+ binds to calmodulin, which consequently binds come myosin light-chain kinase (MLCK). The active calmodulin-MLCK complicated then phosphorylates the myosin II regulatory light chain, convert myosin native an inactive (more...)


Unconventional Myosins

In enhancement to myosin II (“conventional” two-headed myosin), several other types of myosin are discovered in nonmuscle cells. In contrast to myosin II, these “unconventional” myosins do not kind filaments and also therefore are not involved in contraction. Castle may, however, be involved in a range of other kinds of cabinet movements, such together the carry of membrane vesicles and organelles follow me actin filaments, phagocytosis, and extension the pseudopods in amoebae (see figure 11.17).

The best-studied of this unconventional myosins space members of the myosin I family (Figure 11.29). The myosin i proteins save a globular head group that acts together a molecular motor, choose that of myosin II. However, members that the myosin I family members are lot smaller molecules (about 110 kd in mammalian cells) that lack the long tail the myosin II and do not kind dimers. Your tails can instead tie to various other structures, such together membrane vesicles or organelles. The movement of myosin I follow me an actin filament have the right to then transfer its attached cargo. One function of myosin I, debated earlier, is to form the lateral arms that connect actin bundles to the plasma membrane of minister microvilli (see figure 11.16). In this structures, the motor task of myosin I may move the plasma membrane follow me the actin bundles, toward the guideline of the microvillus. Additional functions the myosin I might be in the move of vesicles and organelles follow me actin filaments and in movement of the plasma membrane throughout phagocytosis and also pseudopod extension.


Figure 11.29

Myosin I. Myosin I has a head group comparable to myosin II, however it has actually a comparatively quick tail and also does not kind dimers or filaments. Back it can not induce contraction, myosin I deserve to move follow me actin filaments (toward the add to end), carrying (more...)


In addition to myosins I and II, at the very least 12 other classes that unconventional myosins (III through XIV) have been identified. Several of these unconventional myosins room two-headed prefer myosin II, conversely, others space one-headed choose myosin I. The attributes of most of this unconventional myosins continue to be to it is in determined, however some have actually been plainly shown to play essential roles in organelle motion (myosins V and also VI) and also in sensory features such together vision (myosin III) and also hearing (myosins VI and VII).


Cell Crawling

The crawling movements of cells throughout a surface stand for a basic type of cell locomotion, employed by a wide selection of various kinds that cells. Examples encompass the activities of amoebas, the migrate of embryonic cells during development, the invasion of organization by white blood cell to struggle infection, the hike of cells affiliated in wound healing, and also the spread out of cancer cells during the metastasis of malignant tumors. Comparable types of activity are additionally responsible for phagocytosis and also for the extension of nerve cell processes during advance of the concerned system. All of these activities are based upon the dynamic nature of the actin cytoskeleton, return the detailed mechanisms involved remain to be fully understood.

Cell crawling entails a coordinated cycle that movements, which deserve to be regarded in three stages. First, protrusions such together pseudopodia, lamellipodia, or microspikes (see number 11.17) have to be prolonged from the top edge of the cell (Figure 11.30). Second, these expansions must connect to the substratum across which the cell is migrating. Finally, the trailing edge the the cell have to dissociate indigenous the substratum and also retract right into the cell body.


Figure 11.30

Cell crawling. The crawling movements of cells across a surface deserve to be viewed as 3 stages of combination movements: (1) expansion of the top edge, (2) attachments of the top edge come the substratum, and also (3) retraction of the behind of the cell (more...)


A variety of experiments indicate that extension of the top edge requires the polymerization and crosslinking of actin filaments. Because that example, inhibition the actin polymerization (e.g., by treatment with cytochalasin) block the formation of cell surface protrusions. The regulated turnover of actin filaments, as shown in figure 11.5, leader to the expansion of processes such together filopodia and lamellipodia at the top edge of the cell, and also both cofilin and also Arp2/3 proteins show up to be affiliated in this process. Unconventional myosins may additionally participate in the expansion of procedures at the top edge: Myosin i is forced for pseudopod expansion in the amoeba Dictyostelium and Myosin V for extension of filopodia in neurons.

Following your extension, protrusions native the top edge must connect to the substratum in stimulate to function in cabinet locomotion. Because that slow-moving cells, such as fibroblasts, attachment entails the development of focal distance adhesions (see figure 11.13). Cell moving an ext rapidly, such together amoebas or white blood cells, type more diffuse contacts with the substratum, the molecule composition of i beg your pardon is no known.

The 3rd stage of cabinet crawling, retraction the the rolling edge, is the least understood. The attachments the the rolling edge come the substratum space broken, and also the behind of the cabinet recoils into the cabinet body. The process appears to call for the advancement of tension between the front and also rear that the cell, generating contractile force that eventually pulls the behind of the cabinet forward. This facet of cell locomotion is impaired in mutants of Dictyostelium lacking myosin II, continuous with a function for myosin II in contracting the actin cortex and also generating the force required for retraction of the trailing edge.

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