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Lodish H, Berk A, Zipursky SL, et al. Molecular Cell Biology. 4th edition. New York: W. H. Freeman; 2000.

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Molecular Cell Biology. 4th edition.

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Section 18.5Actin and Myosin in Nonmuscle Cells

Using our discussion of actin and myosin interactions in muscle cells as background, we now examine the function of actin-myosin structures in nonmuscle cells. At first, scientists thought that most cell movements were caused by a contractile mechanism similar to the sliding of actin and myosin filaments in muscle cells. This idea was based on several properties of at least some nonmuscle cells: the ability of cytosolic extracts to undergo contractile-like movements, the presence of actin and myosin II, and the existence of structures similar to muscle sarcomeres both in their organization and in their having ends anchored to the plasma membrane by proteins also found at the muscle Z disk.

Later biochemical studies led to the extraction of myosin I, which does not form thick filaments, from nonmuscle cells. As discussed earlier in the Chapter, 13 different myosins have been identified to date, but researchers have studied only 3 — myosin I, myosin II, and myosin V — in most detail. In this section, we discuss some of the functions of these three myosins in various nonmuscle cells.

Actin and Myosin II Are Arranged in Contractile Bundles That Function in Cell Adhesion

Nonmuscle cells contain prominent bundles of actin and myosin II filaments in the cellular region that contacts the substratum or another cell. When isolated from cells, these bundles contract upon addition of ATP. The contractile bundles of nonmuscle cells differ in two ways from the noncontractile bundles of actin described earlier in this chapter (see Figure 18-5a). First, contractile bundles always are located adjacent to the plasma membrane like a sheet or belt, whereas noncontractile actin bundles form the core of membrane projections (microvilli and filopodia). Second, interspersed among the actin filaments of a contractile bundle is myosin II, which is responsible for the contractility of the bundle. The noncontractile actin bundles in microvilli and filopodia sometimes are associated with myosin I, but myosin II usually is not a major component of these bundles. Despite their ability to contract, contractile bundles probably function primarily in cell adhesion rather than cell movement.

Contractile bundles in epithelial cells are most commonly found as a circumferential belt, which encircles the inner surface of the cell at the level of the adherens junction (Figure 18-35; see Figure 15-23). A circumferential belt resembles a primitive sarcomere in its organization and contains many proteins found in stress fibers and smooth muscle, including vinculin, tropomyosin, and α-actinin. As a complex with the adherens junction, the circumferential belt functions as a tension cable that can internally brace the cell and thereby control its shape. Contraction of the circumferential belt in epithelial cells surrounding a wound seals the gap in the sheet of cells and thus aids in wound healing.

Figure 18-35. The circumferential belt is located near the apical surface of epithelial cells.

Figure 18-35

The circumferential belt is located near the apical surface of epithelial cells. In epithelial tissue, a belt of actin and myosin filaments rings the inner surface of the cell adjacent to the adherens junctions, where cell-cell contacts are maintained. (more...)

As noted early in this chapter, long bundles of actin microfilaments, called stress fibers, lie along the ventral surfaces of cells cultured on artificial (glass or plastic) surfaces (see Figure 18-1b). Fluorescent-antibody techniques reveal that myosin and α-actinin are distributed in alternating patches in stress fibers, much like the pattern of alternating thick filaments and Z bands in muscle sarcomeres. Stress fibers also contain tropomyosin, caldesmon, and the regulatory protein myosin LC kinase, which are found in smooth muscle. The ends of stress fibers terminate at focal adhesions, special structures that attach a cell to the underlying substratum. When stress fibers are separated from focal adhesions by cutting their ends with a laser beam, they contract on addition of ATP, thus demonstrating their contractility.

Although stress fibers are contractile and found in motile cells, several observations suggest that they function in cell adhesion rather than in movement. For one, migrating fibroblasts have few stress fibers during the time of rapid movement; however, once the cells stop migrating, the stress fibers increase in number. Also, when a cultured fibroblast is removed from its substratum, the cell becomes spherical and the stress fibers disappear. If the cell is returned to its substratum, stress fibers reappear within a few hours. In fact, stress fibers may be an artifact caused by culturing cells on glass or plastic surfaces, as they are rarely seen in cells in tissues. Apparently, the adhesion of cells to a substratum induces stresses on the cytoskeleton that cause the random assortment of actin and myosin filaments to align into stress fibers.

In Chapter 22, which covers the integration of cells into tissues, we describe the complex structure of adherens junctions and focal adhesions and how they are attached to contractile bundles.

Myosin II Stiffens Cortical Membranes

We saw earlier that cortical actin networks help support and stiffen the fluidlike plasma membrane. In addition to various actin cross-linking proteins, myosin II also is a component of the cortical cytoskeleton. Two observations support the hypothesis that myosin II molecules act as small tension rods that “tighten up” the cortical actin cytoskeleton. First, genetic and biophysical studies show that the membranes of mutant cells that lack myosin II are deformed more easily than the membranes of normal cells (Figure 18-36a). Second, capping experiments outlined in Fig 18-36b indicate that cortical myosin II is responsible for the movements of some cell-surface proteins. Although cell-surface proteins are normally immobile in the membrane, they will cluster, or “cap,” at one region in the membrane in the presence of antibodies or lectins that bind them. Capping is inhibited in cells lacking myosin II, suggesting that myosin II in the cortex provides the force that aggregates the membrane proteins. In the last section of this chapter, we discuss how myosin-dependent tension in the cortex may contribute to cell locomotion.

Figure 18-36. Myosin stiffening of the cortex.

Figure 18-36

Myosin stiffening of the cortex. (a) In cells containing a normal cytoskeleton, the membrane resists deformation when a bead-topped poker is pushed against it. In a mutant cell lacking myosin II, the membrane is easily deformed when the same force is (more...)

Actin and Myosin II Have Essential Roles in Cytokinesis

Fluorescence microscopy shows that during mitosis actin and myosin II accumulate at the equator of a dividing cell, midway between the poles of the spindle (Figure 18-37a). There they align into a contractile ring, which is similar to a stress fiber or circumferential belt and encircles the cell. As division of the cytoplasm (cytokinesis) proceeds, the diameter of the contractile ring decreases, so that the cell is pinched into two parts by a deepening cleavage furrow. Dividing cells stained with antibodies against myosin I and myosin II show that myosin II is localized to the contractile ring, while myosin I is at the cell poles (Figure 18-37b). This localization indicates that myosin II but not myosin I is involved in cytokinesis.

Figure 18-37. Localization of myosin I and myosin II during cytokinesis.

Figure 18-37

Localization of myosin I and myosin II during cytokinesis. (a) As the spindle poles and aster microtubules separate during mitosis, myosin (blue) and actin (red) in the cortex of the cell assemble into an equatorial contractile ring around the cell. As (more...)

Experiments in which active myosin II was eliminated from the cell demonstrated that cytokinesis is indeed dependent on myosin II (Figure 18-38). In one type of experiment, anti-myosin II antibodies were microinjected into one blastomere of a sea urchin embryo at the two-cell stage. In other experiments, expression of myosin II was inhibited in the Dictyostelium ameba by genetic deletion of the myosin gene or by antisense inhibition of myosin mRNA expression. The results were identical in all cases: cells lacking myosin II became multinucleated because cytokinesis but not karyokinesis (chromosome separation) was inhibited. Without myosin II, the cells failed to assemble a contractile ring, although other events in the cell cycle proceeded.

Figure 18-38. Experimental demonstration that myosin II is required for cytokinesis.

Figure 18-38

Experimental demonstration that myosin II is required for cytokinesis. The activity of myosin II was inhibited either by deleting its gene or by microinjecting anti-myosin II antibodies into a cell. A cell that lacked myosin II was able to replicate its (more...)

Membrane-Bound Myosins Power Movement of Some Vesicles

Among the many movements exhibited by cells, vesicle translocation has been one of the most fascinating to cell biologists. In early studies of the cytoplasm, researchers found that certain particles, now known to be membrane-bounded vesicles, moved in straight lines within the cytosol, sometimes stopping and then resuming movement, at times after changing direction. This type of behavior could not be caused by diffusion because the movement was clearly not random. Therefore, researchers reasoned, there must be tracks, most likely actin filaments or microtubules, along which the particles travel, as well as some type of motor to power the movement.

Unlike cytokinesis, which involves myosin II, other motile processes, such as vesicle transport and membrane movements at the leading edge, possibly involve other myosins and, as discussed in the next chapter, some microtubule motor proteins also. Here we present evidence that some myosins, including myosins I and V, can move along an actin filament while carrying a membrane vesicle as cargo. In these processes, the interaction of actin and myosin differs from that in the sarcomere. We defer discussion of the role of myosin I in directing the movement of the leading edge of cells until the last section of this chapter.

Role of Myosins I and V in Moving Vesicles along Actin Filaments

Studies with amebas provided the initial clues that myosin I participates in vesicle transport. Indeed the first myosin I molecule to be identified and characterized was from these organisms. The cDNA sequences of three myosin I genes have now been identified in Acanthameba, a common soil ameba. Using antibodies specific for each myosin I isoform, researchers found that they are localized to different membrane structures in the cell. For example, myosin IA is associated with small cytoplasmic vesicles. Myosin IC, by contrast, is found at the plasma membrane and at the contractile vacuole, a vesicle that regulates the osmolarity of the cytosol by fusing with the plasma membrane. The introduction of antibodies against myosin IC into a living ameba prevents transport of the vacuole to the membrane; as a result, the vacuole expands uncontrollably, eventually bursting the cell.

Myosin I is also implicated in vesicle transport in vertebrate cells. For example, in intestinal epithelial cells, myosin I co-purifies with vesicles derived from Golgi membranes. The presence of this motor on Golgi membranes suggests myosin I moves membrane vesicles between membrane compartments in the cytoplasm. In addition myosin I serves as a membrane-microfilament linkage in microvilli (see Figure 18-10), another example of a membrane-associated function.

Several types of evidence suggest that myosin V also participates in the intracellular transport of membrane-bounded vesicles. For example, mutations in the myosin V gene in yeast disrupt protein secretion and lead to an accumulation of vesicles in the cytoplasm. Vertebrate brain tissue is rich in myosin V, which is concentrated on Golgi stacks (Figure 18-39). This association with membranes is consistent with the effects of myosin V mutations in mice. Such mutations are implicated in defects in synaptic transmission and eventually cause death from seizures.

Figure 18-39. Immunofluorescence micrograph of astrocytes stained with a reagent specific for myosin V.

Figure 18-39

Immunofluorescence micrograph of astrocytes stained with a reagent specific for myosin V. In these cells, which are found in nervous tissue, myosin V is concentrated in the Golgi stacks (bright central region) and at the tips of membrane processes that (more...)

Myosin-Generated Movements in Cytoplasmic Streaming

Membrane-associated myosin also is critical in the phenomenon of cytoplasmic streaming in large, cylindrical green algae such as Nitella and Chara. In these organisms, the cytosol flows rapidly, at a rate approaching 4.5 mm/min, in an endless loop around the inner circumference of the cell (Figure 18-40a). The rapid flow of cytosol, sustained over millimeter-long distances, is a principal mechanism for distributing cellular metabolites, especially in large cells such as plant cells and amebas. This type of movement probably represents an exaggerated version of the smaller-scale movements exhibited during the transport of membrane vesicles.

Figure 18-40. Cytoplasmic streaming in cylindrical giant algae.

Figure 18-40

Cytoplasmic streaming in cylindrical giant algae. (a) The center of a Nitella cell is filled with a single large water-filled vacuole, which is surrounded by a layer of moving cytoplasm (indicated by blue arrows). A nonmoving layer of cortical cytoplasm (more...)

Close inspection of objects caught in the flowing cytosol, such as the endoplasmic reticulum (ER) and other membrane-bounded vesicles, showed that the velocity of streaming increases from the cell center (zero velocity) to the cell periphery. This gradient in the rate of flow is most easily explained if the motor generating the flow lies at the membrane. In electron micrographs, bundles of actin filaments can be seen aligned along the length of the cell, lying across chloroplasts embedded at the membrane. Attached to the actin bundles are vesicles of the ER network (Figure 18-40b). The bulk cytosol is propelled by myosin attached to parts of the ER lying along the stationary actin filaments. Although the Nitella myosin has not been isolated, it must be one of the fastest known, because the flow rate of the cytosol in Nitella is at least 15 times faster than the rate produced by any other myosin.


  •  In nonmuscle cells, interactions of actin filaments with various myosins are important in such cellular functions as support, cytokinesis, and transport.
  •  Actin filaments and myosin II form contractile bundles that have a primitive sarcomerelike organization and function in cell adhesion. Common examples are the circumferential belt present in epithelial cells and stress fibers found along the ventral surface of cells cultured on plastic or glass surfaces. The latter rarely occur in cells in tissues and may be an artifact.
  •  Interaction of myosin II with cortical actin filaments helps stiffen the plasma membrane, reducing the likelihood of surface deformation.
  •  A transient actin – myosin II contractile bundle (the contractile ring) forms in dividing cells and pinches the cell into two halves during cytokinesis (see Figure 18-37).
  •  Myosins I and V power intracellular translocation of some membrane-limited vesicles along actin filaments. A similar process is responsible for cytoplasmic streaming although the identity of the myosin involved is unknown (see Figure 18-40).
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By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed.

Copyright © 2000, W. H. Freeman and Company.
Bookshelf ID: NBK21562


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