Microtubule Structure

Structure and Function of Microtubules

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. 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 dimers.

Microtubules may work alone, or join with other proteins to form more complex structures called cilia, flagella or centrioles . In this unit we will cover all of these structures. Read the Chapter on Microtubules in Gartner and Hiatt, p 46-49.

Test yourself!! What do you already know about microtubules, cilia, and centrioles?

What is the structural subunit of a microtubule?
What ingredients would you need to make a microtubule in a test tube?
What is the difference between nucleation and elongation?
How could the GTP cap be used to regulate microtubule growth?
How do Microtubule associated proteins help with the functional differentiation of a cell? What are kinesins and dyneins and how do they work?
Why are drugs that prevent microtubule formation useful in the treatment of cancer?

What are cilia and flagella ? What is the role of dynein in the movement of cilia and flagella?
Why are centrioles important to cilia function? What are basal bodies?
How is the internal structure of centrioles and cilia or flagella different?
How do centrioles control the direction of the ciliary beat?
How do centrioles replicate?

Microtubule Structure

Microtubules can be seen in a bundle in this negatively stained preparation. "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. (Taken from Bloom and Fawcett; Textbook of Histology)

The transmission electron micrograph to the right shows the microtubules in longitudinal ultrathin section. Note, the tubulin molecules cannot be visualized in this preparation.

Microtubule Formation

The first stage of formation is called "nucleation". The process requires tubulin, Mg++ and GTP and also proceeds at 37 C. This stage is relatively slow until the microtubule is initially formed. Then the second phase, called "elongation" proceeds much more rapidly.

During "nucleation", an alpha and a beta tubulin molecule join to form a heterodimer.

Then these attach to other dimers to form oligomers which elongate to form protofilaments. Each dimer carries two GTP molecules. However the GTP that appears to function binds to the beta tubulin molecules. When a tubulin molecule adds to the microtubule, the GTP is hydrolyzed to GDP. Eventually the oligomers will join to form the ringed microtubule.

The figure shows that, as the oligomers assemble, they form a series of rings, 25 nm in diameter. In cross section, each ring consists of 13 beads. The rows of beads in longitudinal section are called protofilaments.

In the cell itself, microtubules are formed in an area near the nucleus called the "aster". This is also called the Microtubule Organizing Center (MTOC). Microtubules are polar with a plus end (fast growing) and a minus end (slow growing). Usually the minus end is the anchor point in the MTOC. In this figure, the plus end is shown to the left by the numerous tubulin dimers. This is the end that carries the GTP molecules.

When the GTP is added, it may be broken down to GDP (removing a phosphate group). Breaking down (Hydrolysis) of GTP is not needed for polymerization; microtubules will form normally. However, they will not be able to depolymerize (see below). Thus, the normal role of GTP hydrolysis may be to promote the constant growth of microtubules as they are needed by a cell.

Dynamic instability: Microtubules may vary in their rate of assembly and disassembly. Tubulin half life is nearly a full day, however, the half life of a given microtubule may be only 10 minutes. Thus, they are in a continued state of flux. This is believed to respond to the needs of the cell and is called "dynamic instability". Furthermore, there are regulatory processes that appear to control this in a cell. Microtubule growth would be promoted in a dividing or moving cell. However, microtubule growth would be more controlled in a stable, polarized cell.

One way to regulate further growth would be to put a GTP cap on the growing end of a microtubule to regulate further growth. This happens when the tubulin molecules are added faster than the GTP can be hydrolyzed. This causes the microtubule to become stable and not depolymerize.

How Microtubule Associated Proteins (MAPs) function.

Microtubule associated proteins (MAPs) are tissue and cell type specific. Here are some different types:

Assembly Microtubule associated proteins: They may be high molecular weight proteins (200-300 K) or the tau (20-60 k) proteins. One domain binds to tubulin polymers or unpolymerized tubulin. This speeds up polymerization, facilitates assembly and stabilizes the microtubules. The other end projects out and will bind to vesicles or granules, IF or other MT.

nType I (MAP IA, IB) Projects from microtubules and the other end binds to Intermediate filaments, microtubules, or plasma membrane. Controls spacing of microtubules in the cell. Found in dendrites or axons.

Type II (MAP 2, 4, tau) also found in axons or dendrites. It cross links microtubules with intermediate filaments, plasma membrane or other microtubules. It stabilizes microtubules during growth, function, and cell division. tau organizes microtubules into bundles. MAP4 important for cell division.

Motor Microtubule associated proteins.

This figure shows a 3-D view of a neuron with its processes containing microtubules. At higher magnifications, the vesicles are seen attached to MAPs and moving along the microtubule conveyer belt. The MAPs include kinesins and dynein which "walk" along the microtubules in opposite directions.

The kinesins move the vesicle along towards the plus end and dynein walks towards the minus end. In neurons, as the microtubules grow from the cell body through the processes, the plus end is more peripheral.

These proteins have head regions that bind to microtubules and also bind ATP. The head domains are thus ATPase motors. The tail domain binds to the organelle to be moved. ATP is needed for both binding and movement. Hydrolysis is absolutely essential for movement. It is not known how the energy from ATP breakdown is converted into vectorial transport. Kinesin can bind directly to cargo at its light chain end, however dynein requires a complex of proteins.

Microtubule motility: Experiments in vitro

One can label beads with kinesin or dyneins and watch the direction of movement in a cell at the light microscopic level. What would happen if the beads were simply labeled with "cytoplasmic extract"? This cartoon shows the motility process in vitro. The tubule is moving along a negatively charged glass surface and the vesicle moves along the tubule.

The above electron micrograph shows microtubules in cross section with the MAP bridge. The arrows point to bridges between microtubules. The star points to a MAP bridge to the vesicle. In summary, MAPs accelerate polymerization, serve as "motors" for vesicles and granules, and essentially control cell compartmentation.

Drugs that disrupt microtubules.

Colchicine, colcemid, and nocadazol inhibit polymerization by binding to tubulin and preventing its addition to the plus ends. The figure to the right shows this inhibition by colchicine (red). Vinblastine and vincristine aggregate tubulin and lead to microtubule depolymerization. Taxol stabilizes microtubules by binding to a polymer.

Learn how microtubules are organized in cilia and flagella.

	Learn about Intermediate filaments.
	Learn about Actin filaments.