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Definição e significado de Microtubule

Definição

microtubule (n.)

1.a microscopically small tubule

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Dicionario analógico

microtubule (n.)

tube, tubule[Hyper.]


Wikipedia

Microtubule

                   


Microtubules are a component of the cytoskeleton. These rope-like polymers of tubulin can grow as long as 25 micrometers and are highly dynamic. The outer diameter of microtubule is about 25 nm. Microtubules are important for maintaining cell structure, providing platforms for intracellular transport, forming the spindle during mitosis, as well as other cellular processes.[1] There are many proteins that bind to the microtubule, including motor proteins such as kinesin and dynein, severing proteins like katanin, and other proteins important for regulating microtubule dynamics.[2] [3]

Contents

  Structure

Microtubules are long, hollow cylinders made up of polymerised α- and β-tubulin dimers.

Tubulin dimers polymerize end to end in protofilaments which are the building block for the microtubule structure. 13 protofilaments associate laterally to form a single microtubule and this structure can then extend by addition of more protofilaments to generate the long, hollow, cylindrical structure of a microtubule. Microtubules can extend in length indefinitely.

The lateral association of the protofilaments generates 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. The number of protofilaments can vary; microtubules made up of 14 protofilaments have been seen in vitro.

Microtubules have a distinct polarity which is important for their biological function. 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 the (−) and (+) ends, 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, has only α subunits exposed. Elongation of microtubules typically only occurs from the (+) end.

  Organization within cells

Microtubules are part of a structural network (the cytoskeleton) within the cell's cytoplasm. The primary role of the microtubule cytoskeleton is mechanical. However, in addition to structural support, microtubules also take part in many other processes. A microtubule is capable of growing and shrinking in order to generate force, and there are also motor proteins that allow organelles and other cellular factors to be carried along a microtubule. This combination of roles makes microtubules important for organising cell layout.

  Microtubule nucleation

Microtubules are typically nucleated and organized by dedicated organelles called microtubule-organizing centers (MTOCs). MTOCs associated with the base of a eukaryotic cillium or flagellum are typically termed basal bodies, otherwise they are called centrioles. In many cell types microtubules are primarily nucleated at MTOCs however there are also many exceptions to this rule.

  Cilia and flagella

Microtubules have a major structural role in eukaryotic cillia and flagella. Cillia and flagella are also notable in that they always extend directly from a MTOC, in this case termed the basal body. The action of motor proteins on the neighbouring microtubule strands which run along a cillia or flagellum allow the organelle to bend and generate force for swimming, moving extracellular material, and other roles

Note that prokaryotes do not possess tubulin or microtubules. Prokaryote (both bacterial and archeal) flagella are entirely different in structure to the eukaryotic flagellum.

  Organisation during cell division

  Mitotic spindle

A notable structure involving microtubules is the mitotic spindle used by most eukaryotic cells to segregate their chromosomes correctly during cell division.

The process of mitosis is facilitated by a subgroup of microtubules known as astral microtubules, defined as a microtubule originating from the centrosome that does not connect to a kinetochore. Astral microtubules develop in the actin skeleton and interact with the cell cortex to aid in spindle orientation. They are organized into radial arrays around the centrosomes. The turn-over rate of this population of microtubules is higher than that of any other population. Astral microtubules function in concert with specialized dynein motors, which are oriented with the light chain portion attached to the cell membrane and the dynamic portion attached to the microtubule. This allows for dynein contraction to pull the centrosome toward the cell membrane, thus assisting in cytokinesis.

Astral microtubules are not required for the progression of mitosis, but they are required to ensure the fidelity of the process; they are required for the correct positioning and orientation of the mitotic spindle apparatus. They are also involved in determination of cell division site based on the geometry and polarity of the cells. The maintenance of astral microtubules is dependent on the integrity of centrosome. It is also dependent on several microtubule-associated proteins such as EB1 and Adenomatous Polyposis Coli (APC).

  Midbody

Cell division in a typical eukaryote finishes with the generation of a final cytoplasmic bridge between the to daughter cells termed the midbody. This structure is rich in microtubules and is built up of microtubules which originally made part of the spindle.

  Nucleation and growth

As described above microtubules are often nucleated at a dedicated microtubule-organizing center. Contained within the MTOC is another type of tubulin, γ-tubulin, which is distinct from the α and β subunits, which compose the microtubules themselves. The γ-tubulin combines with several other associated proteins to form a circular structure known as the "γ-tubulin ring complex" (γ-TuRC). This complex acts as a scaffold for α/β tubulin dimers to begin polymerization; it acts as a cap of the (−) end while microtubule growth continues away from the MTOC in the (+) direction.

  Cells lacking MTOCs

Some cell types, such as plant cells, do not contain MTOCs. In these cells, microtubules are nucleated from discrete sites in the cytoplasm. Other cell types, such as trypanosomatid parasites, have a MTOC but it is permanently found at the base of a flagellum. Nucleation of microtubules for structural roles and for generation of the mitotic spindle are not from a canonical centriole-like MTOC. The regulation of the microtubule cytoskeleton in these cells is an intense area of study.

  Dynamic instability

Dynamic instability refers to the coexistence of assembly and disassembly at the (+) end of a microtubule. The microtubule can switch between the growing and shrinking phases dynamically at this region.[4] During polymerization, both the α- and β-subunits of the tubulin dimer are bound to a molecule of GTP. While the GTP bound to α-tubulin is stable, the GTP bound to β-tubulin may be hydrolyzed to GDP shortly after assembly. The kinetics of GDP-tubulin are different from those of GTP-tubulin; GDP-tubulin is prone to depolymerization. A GDP-bound tubulin subunit at the tip of a microtubule will fall off, though a GDP-bound tubulin in the middle of a microtubule cannot spontaneously pop out. Since tubulin adds onto the end of the microtubule only in the GTP-bound state, there is a cap of GTP-bound tubulin at the tip of the microtubule, protecting it from disassembly. When hydrolysis catches up to the tip of the microtubule, it begins a rapid depolymerization and shrinkage. This switch from growth to shrinking is called a catastrophe. GTP-bound tubulin can begin adding to the tip of the microtubule again, providing a new cap and protecting the microtubule from shrinking. This is referred to as "rescue."[5]

Dynamic Instability

In vivo microtubule dynamics vary considerably. Assembly, disassembly, and catastrophe rates depend on which microtubule-associated proteins (MAPs) are present.

  Chemical effects on microtubule dynamics

Microtubule dynamics can also be altered by drugs.

  • For example, the cancer-fighting taxane class of drugs paclitaxel [taxol] and docetaxel block dynamic instability by stabilizing GDP-bound tubulin in the microtubule. Thus, even when hydrolysis of GTP reaches the tip of the microtubule, there is no depolymerization and the microtubule does not shrink back.
  • Eribulin binds to the (+) growing end of the microtubules.

  Motor proteins

In addition to movement generated by the dynamic instability of the microtubule itself, the fibers are substrates along which motor proteins can move. The major microtubule motor proteins are kinesin, which moves toward the (+) end of the microtubule, and dynein, which moves toward the (−) end.

  Postulated role in consciousness

In their controversial Orch-OR theory of consciousness, Roger Penrose and Stuart Hameroff postulate that microtubules in neurons conduct quantum-level manipulations of matter, which produces consciousness, based partially on some observations of Gamma Synchrony that indicate that information may propagate through the brain much faster than a chemically mediated neural network would physically permit. Max Tegmark disputes the relevance of these observations, and the matter remains open to debate. David Chalmers[6] argues that quantum theories of consciousness suffer from the same weakness as more conventional theories. Just as he argues that there is no particular reason why particular macroscopic physical features in the brain should give rise to consciousness, he also thinks that there is no particular reason why a particular quantum feature, such as the EM field in the brain, should give rise to consciousness, either. While at least one researcher claims otherwise, Jeffrey Gray states in his book Consciousness: Creeping up on the Hard Problem, that tests looking for the influence of electromagnetic fields on brain function have been universally negative in their result.[7]

  Additional images

  External links

  References

  1. ^ Desai A.; and Mitchison TJ; (1997). "Microtubule polymerization dynamics.". Annu Rev Cell Dev Biol 13: 83–117. DOI:10.1146/annurev.cellbio.13.1.83. PMID 9442869. 
  2. ^ Vale RD (Feb 2003). "The molecular motor toolbox for intracellular transport.". Cell 112 (4): 467–80. DOI:10.1016/S0092-8674(03)00111-9. PMID 12600311. 
  3. ^ Howard J; Hyman AA; (Feb 2007). "Microtubule polymerases and depolymerases.". Curr Opin Cell Biol 19 (1): 31–5. DOI:10.1016/j.ceb.2006.12.009. 
  4. ^ Karp, Gerald (2005). Cell and Molecular Biology: Concepts and Experiments. USA: John Wiley & Sons. p. 355. ISBN 0-471-46580-1. 
  5. ^ Mitchison T, Kirschner M (15–21 November 1984). "Dynamic instability of microtubule growth". Nature 312 (5991): 237–42. DOI:10.1038/312237a0. PMID 6504138. 
  6. ^ David Chalmers. The Conscious Mind: In Search of a Fundamental Theory. ISBN 0-19-510553-2. 
  7. ^ Jeffrey Gray (2004). Consciousness: Creeping up on the Hard Problem. Oxford University Press. ISBN 0-19-852090-5. 

   
               

 

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