Cell shapes can sometimes change, and there appear to be rapid movements of structures inside cells. These phenomena involve a meshwork of rods within cells that can be seen in electron microscopy. Experimentation showed that this meshwork—called the cytoskeleton—fills several important roles:

There are three components of the eukaryotic cytoskeleton: microfilaments (smallest diameter), intermediate filaments, and microtubules (largest diameter). These filaments have very different functions.

MICROFILAMENTS Microfilaments can exist as single filaments, in bundles, or in networks. They are about 7 nm in diameter and up to several micrometers long. Microfilaments have two major roles:

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Microfilaments are assembled from monomers of actin, a protein that exists in several forms and has many functions, especially in animals. The actin found in microfilaments (which are also known as *actin filaments) has distinct ends, designated “plus” and “minus.” These ends permit actin monomers to interact with one another to form long, double helical chains (Figure 5.14A). Within cells, the polymerization of actin into microfilaments is reversible, and the microfilaments can disappear from cells by breaking down into monomers of free actin. Special actin-binding proteins mediate these processes.

In non-muscle cells, actin filaments are associated with localized changes in cell shape. For example, microfilaments are involved in the flowing movement of the cytoplasm called cytoplasmic streaming, in amoeboid movement, and in the “pinching” contractions that divide an animal cell into two daughter cells. Microfilaments are also involved in the formation of cellular extensions called pseudopodia (pseudo, “false,” + podia, “feet”) that enable some cells to move (Figure 5.15). As you will see in Key Concept 41.3, cells of the immune system must move toward other cells during the immune response.

In some cell types, microfilaments form a meshwork just inside the cell membrane. Actin-binding proteins then cross-link the microfilaments to form a rigid netlike structure that supports the cell. For example, microfilaments support the tiny microvilli that line the human intestine, giving it a larger surface area through which to absorb nutrients (Figure 5.16).

INTERMEDIATE FILAMENTS There are at least 50 different kinds of intermediate filaments, many of them specific to a few cell types. They generally fall into six molecular classes (based on amino acid sequence) that share the same general structure. One of these classes consists of fibrous proteins of the keratin family, which also includes the proteins that make up hair and fingernails. Intermediate filaments are tough, ropelike protein structures 8–12 nm in diameter (Figure 5.14B). They are more permanent than the other two types in that they do not continually form and re-form, as the microtubules and microfilaments do.

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Intermediate filaments have two major structural functions:

MICROTUBULES The largest-diameter components of the cytoskeletal system, microtubules, are long, hollow, unbranched cylinders about 25 nm in diameter and up to several micrometers long. Microtubules have two roles in the cell:

Microtubules are assembled from dimers–two similar molecules in a complex–of the protein tubulin, called α-tubulin and β-tubulin. Thirteen chains of tubulin doublets surround the central cavity of the microtubule (Figure 5.14C; see also Figure 5.17B).

α-tubulin + β-tubulin → doublet – polymer of doublets
(microtubule)

As in microfilaments, the two ends of a microtubule are different: one is designated the “plus” end and the other the “minus” end. Tubulin dimers can be rapidly added or subtracted, mainly at the plus end, lengthening or shortening the microtubule.

Many microtubules appear to radiate from a region of the cell called the microtubule organizing center. Tubulin polymerization results in a rigid structure, and tubulin depolymerization leads to its collapse.

Microtubule ↔ tubulin doublets

The capacity to change length rapidly makes microtubules dynamic structures: they are readily adapted for new purposes in the cell. For example, by disassembly and reassembly, microtubules can move to new parts of the cell and assemble new structures needed for cell division. Microtubules from all eukaryotes have this dynamic property, indicating that it is evolutionarily advantageous over a static, unchanging structure.

In plants, microtubules act as a framework for the assembly of cellulose and help orient the cellulose fibers of the cell wall (see Figure 3.18). Electron micrographs of plants frequently show microtubules lying just inside the cell membranes of cells that are forming or extending their cell walls. If the orientation of these microtubules is altered experimentally, it leads to a similar change in the cell wall and a new shape for the cell.

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Microtubules serve as tracks for motor proteins, specialized molecules that use cellular energy to change their shapes and move. Motor proteins bind to and move along the microtubules, carrying materials from one part of the cell to another. Microtubules are also essential in distributing chromosomes to daughter cells during cell division. Because of this, drugs such as vincristine and taxol, which disrupt microtubule dynamics, also disrupt cell division. These drugs are useful for treating cancer, where cell division is excessive.

CILIA AND FLAGELLA Microtubules and their associated proteins line the interior of certain movable appendages on eukaryotic cells: the cilia (Figure 5.17A) and flagella. Many cells have one or the other of these appendages, which form from projections of the cell membrane:

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In cross section, a typical cilium or eukaryotic flagellum is surrounded by the cell membrane and contains a “9 + 2” array of microtubules. As Figure 5.17B shows, nine fused pairs of microtubules form an outer cylinder, and one pair of unfused microtubules runs up the center. Each doublet is connected to the center of the structure by a radial spoke. This structure is essential to the bending motion of both cilia and flagella. How does this bending occur?

The motion of cilia and flagella results from the sliding of the microtubule doublets past one another. This sliding is driven by the motor protein dynein, which, like other motor proteins, works by undergoing reversible shape changes that require chemical energy. Dynein molecules bind between two neighboring microtubule pairs, and as the dynein molecules change shape, the pairs move past one another (Figure 5.18). Another protein, nexin, cross-links the pairs and appears to limit how far the doublets can slide. This causes the cilium or flagellum to bend.

Question

Nexin links cause cilia and flagella to bend when microtubule doublets try to slide past one another. Absence of nexin would result in reduced flagella and ciliary function. This is called immotile cilia syndrome.

Other motor proteins, including kinesin, carry protein-laden vesicles or other organelles from one part of the cell to another (Figure 5.19). These proteins bind to the organelle and “walk” it along a microtubule by a repeated series of shape changes. Recall that *microtubules are directional, with a plus end where there is a net addition of tubulin and a minus end where there is removal. Cytoplasmic dynein (which has a different role than the one found in cilia and flagella) moves organelles attached to it toward the minus end, whereas kinesin moves them toward the plus end (see Figure 5.14). The ability of microtubules to carry vesicles is used by some fish and amphibians to avoid predators. Motor proteins carrying pigment vesicles aggregate them in the center of the cell in response to environmental or nervous system stimuli. This makes the animal appear lighter. When the stimuli are removed, other motor proteins move the vesicles along microtubules to a more even distribution, restoring a darker hue.