Name: microtubule growth and shrinkage
Uploaded: 2021-04-07T12:12:01.000Z
Duration: 2 min 17 s
Channel: JoVE

Nas células eucarióticas, durante a divisão celular, os microtúbulos formam os principais componentes do fuso mitótico e são necessários para a segregação cromossômica.
Os microtúbulos são extremamente dinâmicos. Os microtúbulos individuais crescem, encolhem e alternam rapidamente entre as fases de crescimento e encurtamento. Os microtúbulos exibem instabilidade dinâmica, a mudança imprevisível entre crescimento e encolhimento.
A mudança do crescimento para o encolhimento é chamada de catástrofe e a mudança do encolhimento para o crescimento é chamada de resgate. A qualquer momento, um grupo de microtúbulos está se montando ativamente, enquanto outros estão se desmontando rapidamente.
Os microtúbulos nucleam e crescem pela polimerização de ponta a ponta de heterodímeros de tubulina ligados ao GTP. O heterodímero de tubulina compreende uma subunidade alfa e beta.
A subunidade beta-tubulina está ligada a uma forma hidrolisável de GTP. A hidrólise do GTP em GDP desestabiliza a estrutura dos microtúbulos. A estrutura se espalha na ponta e o efeito se propaga para baixo, causando a despolimerização do microtúbulo.
Uma variedade de proteínas reguladoras controla a dinâmica dos microtúbulos. Várias proteínas associadas a microtúbulos ou MAPs promovem a estabilidade dos microtúbulos, enquanto várias outras proteínas, os fatores de catástrofe, desestabilizam os microtúbulos. As células alteram a atividade das proteínas reguladoras para alterar a dinâmica dos microtúbulos dependendo da fase do ciclo celular.
Por exemplo, durante a interfase, a maioria das células animais contém uma matriz citoplasmática de microtúbulos longos irradiando de um único centrossoma. À medida que as células fazem a transição para a fase mitótica e os centrossomas duplicados se movem em direção aos pólos opostos, a instabilidade dos microtúbulos aumenta.
A instabilidade dos microtúbulos facilita a formação de uma matriz densa e dinâmica de microtúbulos mitóticos, contribuindo para a formação do fuso.

<ol>
	<li>de Forges, H., Bouissou, A. e Perez, F., 2012. Interação entre a dinâmica dos microtúbulos e a organização intracelular. O jornal internacional de bioquímica e biologia celular, 44 (2), pp.266-274. https://doi.org/10.1016/j.biocel.2011.11.009</li>
	<li>Burbank, K.S. e Mitchison, T.J., 2006. Instabilidade dinâmica dos microtúbulos. Biologia Atual, 14(16), pp. R516-R517. https://doi.org/10.1016/j.cub.2006.06.044</li>
	<li>Vaart, B.V.D., Akhmanova, A. e Straube, A., 2009. Regulação da instabilidade dinâmica dos microtúbulos. Transações da Sociedade Bioquímica, 37(5), pp.1007-1013. https://doi.org/10.1042/BST0371007</li>
</ol>

instabilidade dos microtúbulos

Microtubule Instability

Os microtúbulos são filamentos cilíndricos ocos com diâmetro de aproximadamente 25 nm e comprimento que varia de 200 nm a 25 μm. As subunidades de tubulina ligadas a GTP formam heterodímeros αβ para a montagem de microtúbulos. Esses blocos de construção centrais interagem longitudinalmente, polimerizando-se em protofilamentos. Os protofilamentos então interagem entre si por meio de forças de ligação laterais para formar microtúbulos cilíndricos estáveis. Esses filamentos cilíndricos são dinâmicos à medida que passam por repetidas montagens e desmontagens. Esta instabilidade dinâmica característica pode ser encontrada tanto in vivo como in vitro. <h4>Instabilidade Dinâmica</h4> Os microtúbulos individuais podem alongar-se e encolher simultaneamente nas extremidades opostas em um determinado momento. O fato de um microtúbulo estar crescendo ou diminuindo é determinado por suas taxas de catástrofe e resgate. A catástrofe ocorre quando um microtúbulo em crescimento começa a encurtar rapidamente. O resgate é a mudança de um microtúbulo encolhido para se alongar rapidamente. A taxa de hidrólise do GTP ligado à β-tubulina é um fator primário que determina a instabilidade dinâmica. Na célula, tanto as subunidades livres da tubulina quanto suas formas αβ-heterodiméricas estão presentes no pool citoplasmático. A polimerização das subunidades da tubulina inicia quando as subunidades αβ-heterodiméricas ligadas ao GTP estão acima de uma concentração limite, referida como a concentração crítica para a polimerização dos microtúbulos. A β-tubulina existe em duas formas. A β-tubulina ligada a GTP ou forma T é responsável pelo alongamento e estrutura linear estável dos microtúbulos. Em contraste, a β-tubulina ligada ao PIB ou a forma D favorece a desmontagem dos microtúbulos. As β-tubulinas ligadas ao GTP na extremidade de crescimento atuam como uma capa, evitando a curvatura do protofilamento e promovendo o alongamento. Após a hidrólise do GTP, a conformação da β-tubulina é ligeiramente alterada, resultando na curvatura do protofilamento. Esta curvatura facilita a ligação de proteínas desestabilizadoras como estamina e cinesina-13 para remover os heterodímeros de αβ-tubulina. <h4>Fatores que regulam a instabilidade</h4> As proteínas associadas aos microtúbulos ou MAPs são reguladores críticos da instabilidade dinâmica dos microtúbulos. Os MAPs são amplamente classificados como estabilizadores e desestabilizadores com base na sua função na dinâmica dos microtúbulos. Os MAPs estabilizadores ligam-se aos microtúbulos para reduzir o evento catastrófico e promover o alongamento. Por outro lado, os desestabilizadores unem-se para promover a catástrofe. Os MAPs estabilizadores são dominantes durante a interfase e nos microtúbulos axonais e dendríticos dos neurônios, promovendo montagens estáveis. Durante a mitose, os MAPs desestabilizadores são mais comuns. Esses MAPs são responsáveis pela segregação cromossômica e pela desmontagem da malha do citoesqueleto para a divisão celular.

Instabilidade dos Microtúbulos

Os microtúbulos são filamentos cilíndricos ocos com um diâmetro de aproximadamente 25 nm e um comprimento que varia de 200 nm a 25 μm.

Education

JoVE Core

Molecular Biology

Biologia Molecular

Unit 2

Unidade 2

Cell Division

Divisão Celular

KEY TERMS AND CONCEPTS

TERMOS-CHAVE E CONCEITOS

microtubule growth and shrinkage

Microtubules are hollow, cylindrical filaments in the cell. They are about 25 nm wide and can range from 200 nm to 25 μm in length. Their main building blocks are GTP-bound tubulin subunits that form αβ-heterodimers, which then assemble into protofilaments. These protofilaments join by lateral bonds to create stable microtubule cylinders.

Microtubules are highly dynamic because they repeatedly assemble and break down. They can even grow on one end while shrinking on the other at the same time. This behavior is called dynamic instability. It is seen both in living cells and in vitro.

Whether a microtubule grows or shortens depends on catastrophe and rescue. Catastrophe is the rapid switch from growth to shrinkage. Rescue is the switch from shrinkage back to growth. The rate of GTP hydrolysis on β-tubulin is a major factor that controls this behavior.

Cells contain both free tubulin subunits and αβ-heterodimers in the cytoplasmic pool. Polymerization starts when GTP-bound αβ-heterodimers rise above the critical concentration for microtubule polymerization. β-tubulin also exists in two forms. The GTP-bound T-form supports elongation and a stable, straight microtubule. The GDP-bound D-form favors disassembly.

At the growing end, GTP-bound β-tubulin forms a cap that helps block protofilament bending and supports elongation. When GTP is hydrolyzed, β-tubulin changes shape slightly and protofilaments curve. This curved state helps destabilizing proteins such as stathmin and kinesin-13 bind and remove αβ-tubulin heterodimers.

Microtubule-associated proteins, or MAPs, also regulate dynamic instability. Some MAPs stabilize microtubules, while others promote destabilization. Stabilizer MAPs reduce catastrophe and support elongation. They are common during interphase and in the axonal and dendritic microtubules of neurons, where stable structures are needed. During mitosis, destabilizer MAPs are more common. They help chromosome segregation and the breakdown of the cytoskeletal mesh for cell division.

Microtubule Growth and Shrinkage

mitosis and cytokinesis

Mitosis and Cytokinesis Stages

Mitosis and cytokinesis are the two main steps of cell division. Mitosis is the process that divides the nucleus, while cytokinesis splits the rest of the cell. During mitosis, the cell’s genetic material is separated into two sets. This helps form two new nuclei with the same genetic information. Cytokinesis then completes division by separating the cytoplasm, which creates two daughter cells. These stages work together to make cell division possible in growing and repairing tissues.

duplication of chromatin structure

Chromatin Rebuilding During DNA Replication

Chromatin rebuilding during DNA replication helps cells copy DNA and pass on chromosome structure to daughter cells. During cell division, the genome must be disrupted and then reassembled across the whole chromosome. This process allows chromatin to be inherited, rebuilt, and maintained in new cells. The basic unit of chromatin is the nucleosome. A nucleosome contains DNA wrapped around an octamer of histone proteins, with short linker DNA between neighboring nucleosomes. The histone proteins...

cohesins

Cohesins in Meiosis and Genetic Disorders

Cohesin protein complexes act like molecular glue that keeps sister chromatids together. They are important in both mitosis and meiosis. In mitosis, cohesin complexes are removed from chromosomes before anaphase begins. Meiosis has two rounds of chromosome separation. Meiosis I separates homologous chromosomes, while Meiosis II separates sister chromatids. This process produces four daughter cells. The Meiosis I cohesin complex is made of four subunits: Smc1, Smc3, Rec8, and Scc3. Rec8...

condensins

How Condensins Shape Mitotic Chromosomes

Condensins shape chromosomes during mitosis, the stage when a cell divides. These large protein complexes use ATP, the cell’s energy molecule, to help turn loose post-interphase DNA into compact, separate chromosomes. They organize and segregate chromosomal DNA so it can be passed on correctly. Plant and animal cells contain two condensin complexes: condensin I and condensin II. Each complex has five subunits. Two are SMC, or Structural Maintenance of Chromosomes, subunits. One is a kleisin...

the mitotic spindle

How the Mitotic Spindle Separates Chromosomes

The mitotic spindle is a microtubule-based structure that helps separate chromosomes during cell division. It forms in eukaryotic cells and moves sister chromatids to opposite ends of the parent cell. This leaves each new cell nucleus with the correct genetic material. A bipolar spindle has two poles, and that shape is important for chromosome segregation. In many animal cells, two centrosomes are present when mitosis begins. Each centrosome helps organize a circular set of microtubules at one...

centrosome duplication

Centrosome Duplication in the Cell Cycle

Centrosome duplication is a key part of the animal cell cycle. The centrosome is the main microtubule organizing center, or MTOC, in animal cells. It contains two cylindrical centrioles at its core. Each centriole has nine sets of three microtubules held together by proteins. The two centrioles sit at right angles and are surrounded by the pericentriolar matrix, or PCM, a shapeless protein cloud. Duplication starts early in the cell cycle so each daughter cell can receive a centrosome after...

microtubule instability

Microtubules are hollow, cylindrical filaments in the cell. They are about 25 nm wide and can range from 200 nm to 25 μm in length. Their main building blocks are GTP-bound tubulin subunits that form αβ-heterodimers, which then assemble into protofilaments. These protofilaments join by lateral bonds to create stable microtubule cylinders. Microtubules are highly dynamic because they repeatedly assemble and break down. They can even grow on one end while shrinking on the other at the same time.

spindle assembly

How the Mitotic Spindle Forms

The mitotic spindle forms through centrosome-mediated, chromatin-mediated, and microtubule-mediated pathways. These pathways often work at the same time. Together, they build a strong spindle apparatus for cell division. In most cells, centrosomes are the main sites where microtubules begin to grow. During the G2 to prophase transition, centrosomes mature and make more microtubules. The growing microtubules extend from both centrosomes, and their plus-ends search for chromosomes by attaching...

attachment of sister chromatids

Kinetochore-Microtubule Binding in Mitosis

Kinetochore-microtubule binding helps chromosomes line up and separate during mitosis. After the nuclear envelope breaks down, condensed chromosomes are exposed to the bipolar microtubules of the mitotic spindle. The kinetochore is a large, disc-shaped protein complex at the centromere region of each sister chromatid, and it serves as the microtubule binding site. Usually, the plus-end of a single microtubule inserts into the kinetochore. In some cases, a kinetochore first makes lateral...

forces acting on chromosomes

Chromosome Movement During Mitosis

Chromosome movement during mitosis depends on several small forces that act at different times and positions in the cell. In prometaphase, these forces help chromosomes move to the equatorial plane and line up at the metaphase plate. Because the forces change with location and stage, chromosome behavior also changes as mitosis continues. Microtubules and motor proteins create two main kinds of force on chromosomes: poleward force and anti-poleward force, also called polar-ejection force.

separation of sister chromatids

Anaphase Start and Chromatid Release

Anaphase starts when sister chromatids are released from one another and pulled toward opposite spindle poles. Before that happens, cohesion along the chromosomal arms is reduced from prophase to metaphase, which helps the sister chromatids resolve. A residual cohesin link still holds them together until the metaphase-to-anaphase transition. This leftover connection helps prevent early separation and reduces the risk of aneuploidy, or the wrong number of chromosomes, in daughter cells. At the...

the spindle assembly checkpoint

How Cells Safeguard Chromosome Separation

The spindle assembly checkpoint helps cells safeguard chromosome separation during anaphase. It acts as a molecular surveillance system. The checkpoint checks whether the steps needed for chromosome segregation are complete before the cell moves on. Many proteins work together to control this checkpoint. If mutations affect these proteins, a cell may enter anaphase too early. Chromosomes can then separate before the needed steps are finished. When this happens, the daughter cells may receive...

anaphase a and b

Chromatid Movement by Microtubule Forces

Chromatid movement during anaphase depends on microtubule forces at the spindle. Kinetochore microtubules grow from the spindle poles, and their plus ends attach to kinetochores on sister chromatids. The kinetochore protein complex Ndc80 makes low-affinity links with the microtubule plus end.

the contractile ring

How Cells Build the Contractile Ring

Contractile rings are built from microfilaments and help separate daughter cells during cytokinesis. This ring forms as the cell cycle moves forward, but the exact timing and coordination with other cell cycle events are still not fully understood. RhoA, a small GTPase in the Ras superfamily, controls contractile ring assembly and function. The inactive form of RhoA is bound to GDP. A guanine nucleotide exchange factor, or Rho-GEF, is located in the cortex region, where the next cell division...

determining the plane of cell division

How Cells Set the Division Plane

How cells set the division plane during cell division affects daughter cell size and, in many cases, cell fate. The plane of division is perpendicular to the plane of chromosome segregation. Different organisms use different cues to place it, based on their shape and needs. In animal cells, the cleavage furrow forms along the division plane from the cell cortex, the area just below the plasma membrane. The mitotic spindle is the main guide for this position. If the spindle axis changes, the...

the phragmoplast

How Plant Cells Build a New Wall

Plant cells build a new wall during cytokinesis with help from the phragmoplast. This plant structure forms from the central spindle and related cell parts as the cell finishes dividing. In animal cells, the comparable structure is the midbody, but in plants the phragmoplast expands outward to guide new wall formation. The mature phragmoplast has a donut-like shape. It has an outer leading region, a middle transition zone, and an inner lagging region. New microtubules are assembled...

distribution of cytoplasmic content

How Organelles Are Split During Cytokinesis

Cytokinesis splits a cell’s chromosomes and organelles into two daughter cells. Before cell division, organelles grow and divide. They cannot be made from scratch, so each daughter cell must inherit at least one copy to survive. Many details of this organelle sorting process are still not fully known. The cytoplasm contains organelles, salts, proteins, and water. Small organelles such as peroxisomes and lysosomes can be shared by simple diffusion. Because cells contain many copies of these...

anaphase promoting complex

APC Activates Chromosome Separation

The anaphase-promoting complex, or APC, helps control cell cycle progression by destroying specific proteins at the right time. These proteins are tagged with ubiquitin by ubiquitin ligases and then broken down by the proteasome. The SCF (Skp1/Cullin/F-box) complex and APC are two major ubiquitin ligases involved in this process. SCF stays active throughout the cell cycle, but APC turns on during the metaphase-to-anaphase transition. APC becomes active when Cdc20 or Cdh1 binds to it. These...

key terms and concepts

Explore how microtubules grow, shrink, and switch states through catastrophe, rescue, GTP hydrolysis, and MAP control in cells.

Microtubule Growth and Shrinkage | JoVE K12

molecular biology

JoVE Core Biologia Molecular mostra os processos moleculares da célula através de vídeos de animação claros e concisos. Além disso, nos vídeos de Cientistas em ação, a aplicação desses conceitos é demonstrada por meio de experimentos reais em laboratórios modernos.

Microtubule Growth and Shrinkage

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