Microtubule Structure and Function

Microtubules are polymers of the protein tubulin. Tubulin is group of protein monomers that polymerize into protofilaments, which in turn form microtubules. The repeating subunit of a microtubule is a dimer, consisting of one alpha-tubulin and one beta-tubulin. These dimers are arranged end to end into linear tubulin protofilaments. Thirteen parallel protofilaments then associate side to side to form the hollow microtubule. Because the protofilaments are all parallel, the microtubule is polar, with two distinct ends. Microtubule polymerization and depolymerization are faster at the "plus" end than at the "minus" end. The polarity of the microtubule also determines the direction of motion of the motor proteins that move along it. In most cells, the minus ends of microtubules are anchored in microtubule organizing centers (MTOCs) and grow outward from there toward the edge of the cell. The centrosome, a structure in the cytoplasm of animal cells that coordinates the formation of microtubules, which allows cell division to proceed during reproduction, is the main MTOC in animal cells. It is positioned beside the nucleus in nondividing cells. The centrosome contains two centrioles. A centriole is one of a perpendicular pair of barrel-shaped structures in the centrosome, composed of nine triplets of microtubules arranged in a cylinder. The two centrioles are surrounded by the pericentriolar material, which is an amorphous, non-membrane-bounded structure containing proteins that are important for microtubule nucleation and growth. Microtubules play a key role in organizing the cell during mitosis. The centrosome duplicates itself during interphase, the period between cell division cycles. The resulting two centrosomes move to opposite sides of the nucleus at the beginning of mitosis. Microtubules that were present during interphase rapidly depolymerize, and many new microtubules originate from each centrosome. When the nuclear envelope disassembles and the chromatin (a complex of DNA and proteins that are used to form chromosomes) condenses, some microtubules attach to the chromatids, which are the duplicated chromosomes, by proteins at regions called the kinetochores. Other microtubules extend and push the two poles of the mitotic spindle apart. The spindle microtubules and their motor proteins help with chromosome movement when the chromatids first align at the center of the cell, and then separate. After the nuclear envelope re-forms and cytokinesis takes place, the centrosomes of each daughter cell produce a new set of interphase microtubules.

As polar filaments, with minus ends typically anchored in a centrosome or other MTOC, microtubules grow or shrink from their plus ends. Any individual microtubule may be either in a state of slow polymerization (growth) or fast depolymerization (shrinkage) at any given time. Microtubules may switch between these two states rapidly. This cyclic switching between growth and shrinkage is called dynamic instability.

The factor that triggers switching between states in dynamic instability is the binding of guanosine-5′-triphosphate (GTP), which is an energy source for metabolic reactions, to tubulin subunits, and the subsequent hydrolysis of GTP to GDP. When GTP is bound to tubulin, the affinity of tubulin for the microtubule increases, so tubulin is more likely to bind to the microtubule than to depolymerize. After polymerization, GTP bound to the tubulin is hydrolyzed to guanosine diphosphate (GDP), which is a nucleoside diphosphate. As a result, the affinity of tubulin to the microtubule is weakened, and the subunit is more likely to depolymerize. However, if subunits bound to GTP polymerize rapidly, the rate of polymerization outstrips the rate of hydrolysis, and a "GTP cap" of subunits with GTP bound to them builds up. Microtubule growth continues as long as the rate of addition of these subunits is faster than the rate of hydrolysis. When the rate of addition slows, the hydrolysis of GTP to GDP catches up, until the subunits at the plus end of the microtubule are bound to GDP. At this point, the binding affinity drops, and the microtubule begins to depolymerize. As depolymerization continues, the protofilaments of the microtubule fray apart, depolymerization becomes rapid, and the microtubule disassembles. Dynamic instability gives the microtubule cytoskeleton the ability to reorganize quickly. This is especially important in the events of mitosis. It also means that the typical lifetime of a microtubule in a cell is only several minutes. In circumstances where the cell requires more stable microtubule structures, additional microtubule-associated proteins (MAPs) bind to the microtubules, which stabilizes them against depolymerization. Microtubules in the axons (extensions from the neuronal cell body that transmit the signal to receiving cells) and dendrites (extensions from the neuronal cell body that receive input from other cells) of nerve cells are examples of these stabilized microtubule structures.

The filaments of the cytoskeleton can provide structure and support, but in order to transport cellular materials or generate motion, cells rely on motor proteins. A motor protein is a protein, such as dynein or kinesin, that uses the energy released by the hydrolysis of ATP to move along a cytoskeletal filament. Microtubules associate with two families of motor proteins, the kinesins and the dyneins, that can move materials using the microtubules as tracks. A kinesin is a two-headed ATPase motor that moves materials from the minus end of microtubules toward the plus end. Kinesins move in a stepping fashion from one tubulin subunit to the next, maintaining an association with the filament for 100 steps or more. This long association is because of the conformational changes that occur when kinesins bind ATP. A kinesin head bound to tubulin increases its affinity and locks in place upon binding to ATP. The conformational change also positions the second head near the next tubulin subunit on the filament. When the first head hydrolyzes ATP and releases, the second one is ready to attach, this action also deters the motor protein from moving backwards.

Because they move from the minus to the plus ends of microtubules, kinesins can transport cytosolic cargo, such as potassium or sodium ions, from the center of the cell to the periphery. The endoplasmic reticulum has attachments to the opposite and of kinesin motors and acquires its extended, tubular structure by being pulled and stretched away from the nucleus by kinesins traveling along microtubules.

A dynein is a motor protein that moves from the plus end to the minus end of microtubules. Dyneins are much larger ATPase motors than kinesins, with molecular weights of about 1.2 MDa compared to 120 kDa for kinesins. Since the plus ends of microtubules are generally located at the edges of cell, dyneins are used to move materials from the edges of the cell toward the center. They transport a wide variety of cargoes, including endocytotic vesicles, lysosomes, peroxisomes, and viruses. They are also used to position discrete organelles, such as mitochondria, and extended ones, such as the Golgi apparatus, whose membrane chambers are pulled close to the nucleus by the action of dyneins. Dyneins may also exert tension on the nucleus and the centrosome, helping to center them. Dyneins are also involved in cell motility and movement, through their interaction with the microtubules that make up the core structures of cilia and flagella. A flagellum (plural, flagella) is a threadlike tail that allows some cells to move. The structure of flagella differs between prokaryotes and eukaryotes. Flagella are long and whiplike protrusions, up to millimeters in length, and are typically used by protozoa or sperm cells for swimming. A cilium (plural, cilia) is a small, hairlike projection from cells that can be used for motility or to filter air in the respiratory system. Cilia are structurally identical to flagella but are much shorter (measured in microns) and typically occur in large numbers and work in concert to move substances over the cell surface. Both structures are membrane-bound extensions of the cell and contain a microtubule bundle called an axoneme as their core. The axoneme has a $9+2$ structure, with nine doublet microtubules surrounding two single microtubules. Dynein molecules extend between the doublet microtubules and can cause the microtubules to slide past each other. Because the microtubules of the axoneme are anchored to each other in some places and sliding in others, the action of dynein produces a bend. As the bend progresses down the axoneme, it generates the wavelike beating motions of cilia and flagella.