Microtubules are conveyer belts inside the cells. They move vesicles, granules, organelles like mitochondria, and chromosomes via special attachment proteins. They also serve a cytoskeletal role. Structurally, they are linear polymers of tubulin which is a globular protein. These linear polymers are called protofilaments. The figure to the left shows a three dimensional view of a microtubule. The tubulin molecules are the bead like structures. A protofilament is a linear row of tubulin beads.
Microtubules may work alone, or join with other proteins to form more complex structures called cilia, flagella or cen
trioles . In this unit we will cover all of these structures.

Microtubules, the third principal component of the cytoskeleton, are rigid hollow rods approximately 25 nm in diameter. Like actin filaments, microtubules are dynamic structures that undergo continual assembly and disassembly within the cell. They function both to determine cell shape and in a variety of cell movements, including some forms of cell locomotion, the intracellular transport of organelles, and the separation of chromosomes during mitosis.

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external image Microtubule.gif Microtubules
  • are straight, hollow cylinders whose wall is made up of a ring of 13 "protofilaments";
  • have a diameter of about 25 nm;
  • are variable in length but can grow 1000 times as long as they are wide;
  • are built by the assembly of dimers of alpha tubulin and beta tubulin;
  • are found in both animal and plant cells.
  • grow at each end by the polymerization of tubulin dimers (powered by the hydrolysis of GTP), and
  • shrink at each end by the release of tubulin dimers (depolymerization).
However, both processes always occur more rapidly at one end, called the plus end. The other, less active, end is the minus end.
Microtubules participate in a wide variety of cell activities. Most involve motion. The motion is provided by protein "motors" that use the energy of ATP to move along the microtubule.


These straight, hollow cylinders are found throughout the cytoplasm of all eukaryotic cells (prokaryotes don't have them) and carry out a variety of functions, ranging from transport to structural support. Microtubules, which are about 25 nanometers in diameter, form part of the cytoskeleton that gives structure and shape to a cell, and also serve as conveyor belts moving other organelles throughout the cytoplasm. In addition, microtubules are the major components of cilia and flagella, and participate in the formation of spindle fibers during cell division (mitosis). The length of microtubules in the cell varies between 200 nanometers and 25 micrometers, depending upon the task of a particular microtubule and the state of the cell's life cycle.
Microtubule Helical Structure
Microtubule Helical Structure
Microtubules are biopolymers that are composed of subunits made from an abundant globular cytoplasmic protein known as tubulin, as illustrated in Figure 1. Each subunit of the microtubule is made of two slightly different but closely related simpler units called alpha-tubulin and beta-tubulin that are bound very tightly together to form heterodimers. In a microtubule, the subunits are organized in such a way that they all point the same direction to form 13 parallel protofilaments. This organization gives the structure polarity, with only the alpha-tubulin proteins exposed at one end and only beta-tubulin proteins at the other.
By adding or removing globular tubulin proteins, the length of polymeric microtubules can be increased or decreased. Because the two ends of a microtubule are not the same, however, the rate at which growth or depolymerization occurs at each pole is different. The end of a polarized filament that grows and shrinks the fastest is known as the plus end and the opposing end is called the minus end. For all microtubules, the minus end is the one with exposed alpha-tubulins. In an animal cell, it is this end that is located at the centriole-containing centrosome found near the nucleus, while the plus end, comprised of exposed beta-units, is projected out toward the cell's surface. Microtubules are continuously being assembled and disassembled so that tubulin monomers can be transported elsewhere to build microtubules when needed.
Animal Cell Microtubule Network
Animal Cell Microtubule Network
Presented in Figure 2 is a digital image of the microtubule network found in an embryonic mouse cell as seen through a fluorescence optical microscope. In addition to their structural support role, microtubules also serve as a highway system along which organelles can be transported with the aid of motor proteins. For instance, the microtubule network interconnects the Golgi apparatus with the plasma membrane to guide secretory vesicles for export, and also transports mitochondria back and forth in the cytoplasm. Another example is the translocation of vesicles containing neurotransmitters by microtubules to the tips of nerve cell axons. The motor proteins involved in organelle transport operate by altering their three-dimensional conformation using adenosine triphosphate (ATP) as fuel to move back and forth along a microtubule. With each step, the motor molecule releases one portion of the microtubule and grips a second site farther long the filament. Motor proteins, which are grouped into several distinct classes, attach to organelles through specialized receptors.
Since eukaryotic cells greatly depend upon the integrity of microtubules and other cytoskeletal filaments to maintain their structure and essentially to survive, many plants produce natural toxins aimed at disrupting the microtubule network as a means of self-defense. Taxol, for example, is a toxic substance produced by a species of yew trees that increases microtubule polymerization (building a macromolecule) by binding to the filament and stabilizing it. Other natural toxins, such as the colchicine produced by the meadow saffron, destabilize microtubules and hinder their polymerization. Both kinds of events can be fatal to the affected cell, though in some circumstances, this can be beneficial to animals, as demonstrated by taxol, which is commonly used as a cancer medication
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Microtubules, the third principal component of the cytoskeleton, are rigid hollow rods approximately 25 nm in diameter. Like actin filaments, microtubules are dynamic structures that undergo continual assembly and disassembly within the cell. They function both to determine cell shape and in a variety of cell movements, including some forms of cell locomotion, the intracellular transport of organelles, and the separation of chromosomes during mitosis.

Structure: Microtubules can be seen in a bundle in this negatively stained preparation. Recall that negative staining starts by immobilizing the preparation on plastic on an electron microscopic grid. Then heavy metal stain is deposited around the structures, delineating their structure. This preparation may allow you to see the tubulin molecules in the protofilaments.

Function: Microtubules are filamentous intracellular structsure that are responsible for various kinds of movements in all eukaryotic cells. Microtubules are involved in nuceic and cell division, organization of intracellular structure, and intracellular transport, as well as ciliary and flagellar motility. Because the functions of microtubules are so critical to the existence of eukaryotic cells (including our own), it is important that we understand their composition, how they are assembled and disassembled, and how their assembly/disassembly and functions are regulated by cells.

Building blocks of microtubules: All eukaryotic cells produce the protein tubulin, in the usual way. The usual way, of course, is by transcription of genes coding for tubulin to produce messenger RNA, followed by the translation of mRNA by the ribosomes in order to produce protein. Cells maintain at least two types of tubulin, which we call alpha tubulin and beta tubulin. However, it is doubtful that the two types can found in cells as individual proteins.
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Kinesin is an enzyme that hydrolyzes adenosine triphosphate to provide energy to power anterograde [from (-) to (+)] movement along microtubules.
Dyneins are force-generating adenosine triphosphatases (ATPases) that move along eukaryotic microtubules.

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Microtubules are polymers of α- and β-tubulin dimers. The tubulin dimers polymerize end to end in protofilaments. The protofilaments then bundle in hollow cylindrical filaments. Typically, the protofilaments arrange themselves in an imperfect helix with one turn of the helix containing 13 tubulin dimers each from a different protofilament. The image above illustrates a small section of microtubule, a few αβ dimers in length.
Another important feature of microtubule structure is polarity. Tubulin polymerizes end to end with the α subunit of one tubulin dimer contacting the β subunit of the next. Therefore, in a protofilament, one end will have the α subunit exposed while the other end will have the β subunit exposed. These ends are designated (−) and (+) respectively. The protofilaments bundle parallel to one another, so in a microtubule, there is one end, the (+) end, with only β subunits exposed while the other end, the (−) end, only has α subunits exposed.