Microtubule self-repair

The stochastic switching between microtubule growth and shrinkage is a fascinating and unique process in the regulation of the cytoskeleton. To understand it, almost all attention has been focused on the microtubule ends. However, recent research has revived the idea that tubulin dimers can also be exchanged in protofilaments along the microtubule shaft, thus repairing the microtubule and protecting it from disassembly. Here, we review the research describing this phenomenon, the mechanisms regulating the removal and insertion of tubulin dimers, as well as the potential implications for key functions of the microtubule network, such as intracellular transport and cell polarization.     

Microtubule dynamics

A combination of biochemical, structural, and morphological analyses during the last 2 decades has shown that the cytoplasm of a cell is not a disorganized mass of jelly but a highly structured cell compartment formed of a cytoskeleton, one of which principal components are the microtubules. More recently, studies have revealed that microtubule cytoskeleton is not only well organized but highly dynamic, and that microtubule dynamics may be responsible for several cell functions such as chromosome segregation, cell morphogenesis, or intracytoplasmic organization.     

微管成核的研究进展

微管成核是指微管蛋白(tubulin)分子相互作用形成微管组织“核心”的过程,它是微管形成的初始阶段。在一定条件下,微管蛋白溶液中可以发生微管成核现象。γ微管蛋白(γ-tubulin)或多种γ微管蛋白复合体的存在能够加速这一过程。在体内,一般是由γ-TuRC(γ-tubulin ring complex)启动微管的装配。近年来研究发现即使没有γ微管蛋白,机体仍然能够利用某种机制组织微管成核。     

Microtubule nucleation

Microtubule nucleation is the process in which several tubulin molecules interact to form a microtubule seed. Microtubule nucleation occurs spontaneously in purified tubulin solutions, and molecular intermediates between tubulin dimers and microtubules have been identified. Microtubule nucleation is enhanced in tubulin solutions by the addition of gamma-tubulin or various gamma-tubulin complexes. In vivo, microtubule assembly is usually seeded by gamma-tubulin ring complexes. Recent studies suggest, however, that microtubule nucleation can occur in the absence of gamma-tubulin, and that gamma-tubulin may have other cell functions apart from being a major component of the gamma-tubulin ring complex.     

Microtubule functions.

Microtubules, with intermediate filaments and microfilaments, are the components of the cell skeleton which determinates the shape of a cell. Microtubules are involved in different functions including the assembly of mitotic spindle, in dividing cells, or axon extension, in neurons. In the first case, microtubules are highly dynamic, while in the second case microtubules are quite stable, suggesting that microtubule with different physical properties (stability) are involved in different functions. Thus, to understand the mechanisms of microtubule functions it is very important to understand microtubule dynamics. Historically, tubulin, the main component of microtubules, was first characterized as the major component of the mitotic spindle that binds to colchicine. Afterwards, it was found that tubulin is particularly more abundant in brain than in other tissues. Therefore, the roles of microtubules in mitosis, and in neurons, have been more extensively analyzed and, in this review, these roles will be discussed.     

SPIRAL2 Determines Plant Microtubule Organization by Modulating Microtubule Severing

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Microtubule dynamics.

Although compelling evidence has been obtained for heterogeneity in the structure of subunits in microtubules, it has not been possible to prove that this results from the presence of tubulin-GDP and tubulin-GTP in polymers. There are reasons to exclude the existence of even a monolayer of tubulin-GTP subunits at microtubule ends. Dynamic behavior appears to be best accounted for by a mechanism in which tubulin-GDP in microtubules exists in two conformations. The mechanism of microtubule-associated protein binding to microtubules and the role of phosphorylation on this reaction are discussed.     

Anterograde Microtubule Transport Drives Microtubule Bending in LLC-PK1 Epithelial Cells

Microtubules (MTs) have been proposed to act mechanically as compressive struts that resist both actomyosin contractile forces and their own polymerization forces to mechanically stabilize cell shape. To identify the origin of MT bending, we directly observed MT bending and F-actin transport dynamics in the periphery of LLC-PK1 epithelial cells. We found that F-actin is nearly stationary in these cells even as MTs are deformed, demonstrating that MT bending is not driven by actomyosin contractility. Furthermore, the inhibition of myosin II activity through the use of blebbistatin results in microtubules that are still dynamically bending. In addition, as determined by fluorescent speckle microscopy, MT polymerization rarely results, if ever, in bending. We suppressed dynamic instability using nocodazole, and we observed no qualitative change in the MT bending dynamics. Bending most often results from anterograde transport of proximal portions of the MT toward a nearly stationary distal tip. Interestingly, we found that in an in vitro kinesin-MT gliding assay, MTs buckle in a similar manner. To make quantitative comparisons, we measured curvature distributions of observed MTs and found that the in vivo and in vitro curvature distributions agree quantitatively. In addition, the measured MT curvature distribution is not Gaussian, as expected for a thermally driven semiflexible polymer, indicating that thermal forces play a minor role in MT bending. We conclude that many of the known mechanisms of MT deformation, such as polymerization and acto-myosin contractility, play an inconsequential role in mediating MT bending in LLC-PK1 cells and that MT-based molecular motors likely generate most of the strain energy stored in the MT lattice. The results argue against models in which MTs play a major mechanical role in LLC-PK1 cells and instead favor a model in which mechanical forces control the spatial distribution of the MT array.     

Cooperative Accumulation of Dynein-Dynactin at Microtubule Minus-Ends Drives Microtubule Network Reorganization

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Microtubule reconfiguration during axogenesis

When cultured on polylysine, rat sympathetic neurons extend modest lamellae which contain a mass of relatively short non-aligned microtubules. Microtubules display movements, but these movements do not result in any obvious alterations in the overall configuration of the array. Application of a mixture of growth factors called matrigel results in a rapid expansion of the lamellae followed by the outgrowth of axons. Microtubules undergo rapid behavioral changes that result in dramatic alterations in the microtubule array. Microtubules become significantly longer, and extend to the periphery of the lamellae where they invade newly-forming axons. The microtubules align with one another and relative to the cell cortex, and draw together into bundles. Microtubules within a bundle move apart as well, particularly at the tips of developing axons. These observations demonstrate a complexity of microtubule behaviors, some of which can be explained by interactions with actin and/or by forces generated by molecular motor proteins.