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

真核細胞では、細胞分裂中に微小管が有糸分裂紡錘体の主成分を形成し、染色体の分配に必要です。
微小管は非常に動的です。個々の微小管は、成長期と短縮期の間で成長、縮小し、急速に交互に変化します。微小管は、成長と収縮の間の予測不可能な変化である動的不安定性を示します。
成長から縮小への移行は大災害と呼ばれ、縮小から成長への移行はレスキューと呼ばれます。どの時点でも、微小管のグループは活発に組み立てられていますが、他の微小管は急速に分解されています。
微小管は、GTP結合チューブリンヘテロ二量体のエンドツーエンド重合により核を形成し、成長します。チューブリンヘテロ二量体は、アルファおよびベータサブユニットを含む。
β-チューブリンサブユニットは、加水分解型のGTPに結合しています。GTPからGDPへの加水分解は、微小管フレームワークを不安定にします。先端で構造が広がり、その効果が下に伝播し、微小管の解重合を引き起こします。
さまざまな調節タンパク質が微小管のダイナミクスを制御しています。いくつかの微小管関連タンパク質またはMAPは微小管の安定性を促進しますが、他のいくつかのタンパク質、つまり大災害因子は微小管を不安定にします。細胞は、制御タンパク質の活性を変化させ、細胞周期の位相に応じて微小管の動態を変化させます。
例えば、間期には、ほとんどの動物細胞は、単一の中心体から放射状に広がる長い微小管の細胞質配列を含んでいます。細胞が有糸分裂期に移行し、重複した中心体が反対の極に移動すると、微小管の不安定性が増大します。
微小管の不安定性は、有糸分裂微小管の密集した動的配列の形成を促進し、紡錘体の形成に寄与します。

<ol>
	<li>de Forges、H.、Bouissou、A.およびPerez、F.、2012年。微小管動態と細胞内組織化の相互作用。The International journal of biochemistry & cell biology, 44(2), pp.266-274. https://doi.org/10.1016/j.biocel.2011.11.009</li>
	<li>バーバンク、KSおよびミッチソン、TJ、2006年。微小管の動的不安定性。現在の生物学, 14(16), pp.R516-R517. https://doi.org/10.1016/j.cub.2006.06.044</li>
	<li>Vaart、B.V.D.、Akhmanova、A.およびStraube、A.、2009年。微小管の動的不安定性の制御。Biochemical Society Transactions, 37(5), pp.1007-1013. https://doi.org/10.1042/BST0371007</li>
</ol>

微小管の不安定性

Microtubule Instability

微小管は、直径が約25nm、長さが200nmから25μmまで変化する中空の円筒状フィラメントです。GTPに結合したチューブリンサブユニットは、微小管集合のためにαβヘテロ二量体を形成します。これらのコア構成要素は縦方向に相互作用し、重合してプロトフィラメントになります。その後、プロトフィラメントは横方向の結合力によって互いに相互作用して、安定した円筒状微小管を形成します。 これらの円筒状フィラメントは、組み立てと分解を繰り返すことで動的になります。この特徴的な動的不安定性は、in vivoとin vitroの両方で見られます。

動的不安定性

個々の微小管は、特定の時点で反対側の端で同時に伸びたり縮んだりすることがあります。 微小管が成長しているか縮小しているかは、その破局と救済の速度によって決まります。 破局は、成長中の微小管が急速に短縮し始めるときです。救済とは、収縮していた微小管が急速に伸長するように変化することです。βチューブリン結合GTP加水分解の速度は、動的不安定性を決定する主な要因です。

細胞内では、遊離チューブリンサブユニットとそのαβヘテロ二量体形態の両方が細胞質プールに存在します。チューブリンサブユニットの重合は、GTP結合αβヘテロ二量体サブユニットが、微小管重合の臨界濃度と呼ばれる閾値濃度を超えると開始されます。β-チューブリンには2つの形態が存在します。GTP結合βチューブリンまたは T 型は、微小管の伸長と安定した線形構造に関与しています。対照的に、GDP結合βチューブリンまたはD型は微小管の分解を促進します。成長末端のGTP結合βチューブリンはキャップとして機能し、プロトフィラメントの湾曲を防ぎ、伸長を促進します。GTPが加水分解されると、βチューブリンの立体構造がわずかに変化し、プロトフィラメントが湾曲します。この湾曲により、スタスミンやキネシン13などの不安定化タンパク質の結合が促進され、αβチューブリンヘテロ二量体が除去されます。

不安定性を制御する要因

微小管関連タンパク質またはMAPは、微小管の動的不安定性の重要な調節因子です。MAPは、微小管動態における機能に基づいて安定剤と不安定剤に大別されます。安定剤MAPは微小管と結合して、破局事象を軽減し、伸長を促進します。 一方、不安定剤は結合して大惨事を促進します。スタビライザーMAPは、間期中およびニューロンの軸索および樹状微小管で優勢であり、安定した集合を促進します。有糸分裂中は、不安定化MAPがより一般的になります。 これらのMAPは、染色体の分離と細胞分裂のための細胞骨格メッシュの分解に関与します。

微小管は、直径が約25nm、長さが200nmから25μmまで変化する中空の円筒形フィラメントです。

Education

JoVE Core

Molecular Biology

分子生物学

Unit 2

ユニット2

Cell Division

細胞分裂

KEY TERMS AND CONCEPTS

主要用語と概念

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 分子生物学は、明確で簡潔なアニメーション ビデオ レッスンを通じて細胞の分子プロセスを紹介します。さらに、科学者の実際のビデオでは、これらの概念の応用が現代の研究室での実際の実験を通じて実証されています。

Microtubule Growth and Shrinkage

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