Actin and Myosin II Form Contractile Bundles in Nonmuscle Cells

Nonmuscle cells contain prominent contractile bundles composed of actin and myosin II filaments. The contractile bundles of nonmuscle cells, which may be transitory or permanent differ in several ways from the noncontractile bundles of actin described earlier in this chapter (see Figure 19-5). Interspersed among the actin filaments of a contractile bundle is myosin II, which is responsible for their contractility. When isolated from cells, these bundles contract on the addition of ATP. Contractile bundles are always located adjacent to the plasma membrane as a sheet or belt, whereas noncontractile actin bundles form the core of membrane projections (e.g., microvilli and filopodia).

In epithelial cells, contractile bundles are most commonly found as a circumferential belt, which encircles the inner surface of the cell at the level of the adherens junction (see Figure 6-5). .Stress fibers, which are seen along the ventral surfaces of cells cultured on artificial (glass or plastic) surfaces or in extracellular matrices, are a second type of contractile bundle. The ends of stress fibers terminate at integrin-containing focal adhesions, special structures that attach a cell to the underlying substratum (see Figure 6-26). Circumferential belts and stress fibers contain several proteins found in smooth muscle, and both exhibit some orga-

nizational features resembling muscle sarcomeres. Thus, both these structures appear to function in cell adhesion and cell movement.

A third type of contractile bundle, referred to as a contractile ring, is a transient structure that assembles at the equator of a dividing cell, encircling the cell midway between the poles of the spindle. As division of the cytoplasm (cytokinesis) proceeds, the diameter of the contractile ring decreases; so 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, whereas myosin I is at the cell poles (Figure 19-20). This localization indicates that myosin II but not myosin I takes part in cytokinesis.

The results of experiments in which active myosin II is eliminated from the cell demonstrate that cytokinesis is indeed dependent on myosin II (Figure 19-21). In one type of experiment, anti-myosin II antibodies are microinjected into one cell of a sea urchin embryo at the two-cell stage. In other experiments, expression of myosin II is inhibited by deletion of the myosin gene or by antisense inhibition of myosin mRNA expression. In all cases, a cell lacking myosin II replicates to form a multinucleated syncytium because cytokinesis, but not chromosome separation, is inhibited. Without myosin II, cells fail to assemble a contractile ring, although other events in the cell cycle proceed normally.


Cleavage furrow

▲ EXPERIMENTAL FIGURE 19-20 Fluorescent antibodies reveal the localization of myosin I and myosin II during cytokinesis. Fluorescence micrograph of a Dictyostelium ameba during cytokinesis reveals that myosin II (red) is concentrated in the cleavage furrow, whereas myosin I (green) is localized at the poles of the cell. The cell was stained with antibodies specific for myosin I and myosin II, with each antibody preparation linked to a different fluorescent dye. [Courtesy of Y Fukui.]

▲ EXPERIMENTAL FIGURE 19-21 Inhibition of myosin II demonstrates that it is required for cytokinesis. The activity of myosin II can be inhibited either by deleting its gene or by microinjecting anti-myosin II antibodies into a cell. A cell that lacks myosin II is able to replicate its DNA and nucleus but fails to divide; as a result, the cell forms a large, multinucleate syncytium over a period of time. In comparison, an untreated cell during the same period continues to divide, forming a multicellular ball of cells in which each cell contains a single nucleus.

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