Microtubule

Microtubules in the same cell. In A: Tubulin proteins together. In B: you see the Tubulin range alone (red). In C the alpha tubulin alone (green). In D the cell core is shown in blue. Immunohistochemical.

Microtubules (clear lines) within a cell. Only Tubulin beta with antibodies marked for immunofluorescence.

The microtubules are cellular structures formed by protein polymers, 25 nm of outer diameter and about 12 nm inner diameter, with lengths that vary between a few nanometers to micrometers, that originate in the Organizing Center of Microtubules (MTOC in English) and that extend throughout the entire cytoplasm. They are with different characteristics in eukaryotic cells and prokaryotes. They are formed by the polymerization of a dimer of two globular proteins, alpha tubulin and beta tubulin.

Microtubules intervene in various cellular processes that involve displacement of secretion vesicles, organ of organelles, intracellular transportation of substances, as well as in cell division (mitosis and meiosis) and that, together with microfilaments and intermediate filaments, They form the cytoskeleton. In addition, they constitute the internal structure of the cilia and the scourges.

Microtubules are nucleated and organized in the microtubules organizing centers (MTOC), such as centers or basal bodies of cilia and flagella. These organizing centers can have centriles or not.

In addition to collaborating in the cytoskeleton, the microtubules intervene in the transit of vesicles (see dinein or cinemas), in the formation of the mitotic spindle by which eukaryotic cells secrete their chromatids during cell division, and in the Movement of Cilia and Flagelos.

STRUCTURE

Microtubulum. Formed by proteins Tubulin α and β, which are associated to form a dimer, which alternates in a protofilament.

Microtubules are heteropolymers of α- and β-tubulin, which form dollars, which are their structural unit. Dímeros polymerize in 13 protofilaments, which are then added laterally to form hollow cylindrical structures. To polymerize, the presence of diameters is required to a determined minimum concentration called critical concentration, although the process is accelerated by the addition of nuclei, which are elongated.

Microtubules: procariot (bacterial) and eucariot.

An important characteristic of microtubules is their polarity. Polymeriza tubulin by addition of dimers in one or both ends of the microtubule. The addition is by union head with tail, in the formation of protofilaments. Thus, rods are formed of monomers of α and β-tubulin on the wall, which causes a global polarity to the microtubule. Because all the protofilaments of a microtubule have the same orientation, one end is composed of a α-tubulin ring (called end-) and, the opposite, by a ring of β-tubulin (called end + plus end).

Bacteria structure

Bacterial microtubules have a smaller diameter than eukaryotes but have the same basic structure. In the bacteria, tubuline homologists, bacterial tubulin proteins (Btuba in English) and bacterial b bacterial (Btubb) also form microtubules. Bacterial microtubules show protofilaments that are ordered and that have interactions similar to eukaryotes. Bacterial microtubules are composed of only five protofilaments, instead of the 13 present in eukaryotes.

FUNCTION

The polymerization of the microtubules is numbered in an organizing center of microtubules. In them there is a type of tubulin, called γ-tubulin, which acts nucleating the addition of new dimers, with intervention of other regulatory proteins. Thus, the existence of an annular complex of γ -tubulin is considered, always located at the end - of the microtubule.

Dynamic instability

Diagram of tubulin polymerization dynamics.

During polymerization, both tubulin units are bound to a molecule of guanosine triphosphate. GTP plays a structural role in α-tubulin, but is hydrolyzed to GDP in β-tubulin. This hydrolysis modulates the addition of new dimers. Thus, the GTP is hydrolyzed after a lapse of time, which allows that, if the addition of dimers is rapid, a cap of β-tubulin bound to GTP is formed at the (+) end, while, if it is slow, what is exposed is tubulin bound to GDP. Well then: this binding to one or another nucleotide is what determines the speed of polymerization or depolymerization of the microtubule. Thus, a cap at the (+) end with GTP favors elongation, while one with GDP favors depolymerization.

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Now, this process, whether or not to add new monomers, depends on the concentration of αβ-tubulin dimers in the solution; if its concentration is greater than a parameter known as critical concentration (Cc) (which is the dissociation equilibrium constant of the dimers at the end of the microtubule), the microtubule grows, and if it is less, it decreases. And depending on the presence of a GTP or GDP cap, the Cc is different, which defines that the end (+) and (-) have different values, which in turn results in the dynamic activity of the end (+) being higher due to lower specific Cc. The microtubule, therefore, can grow at both ends or at only one, depending on the concentration of αβ-tubulin dimers. The interaction of the (-) end with the MTOC greatly decreases its activity.

MAP

There are other proteins called MAP (Microtubule Associated Protein) or proteins associated with microtubules. They are considered to assist in the assembly of dimers to form microtubules.

MAPs are classified by their molecular weight into two groups:

• Low molecular weight MAP 55-62 kDa

Also called τ(tau) proteins. They coat the microtubule and form junctions with adjacent microtubules.

• High molecular weight MAP 200-1000 kDa

4 different MAP types are known: MAP-1, MAP-2, MAP-3, MAP-4 MAP-1 comprises at least 3 different proteins: A, B and C. C is important in the retrograde transport of vesicles and is called cytoplasmic dynein. MAP-2 are found in the dendrites and body of neurons, where they associate with other filaments. MAP-4 are found in most cells and stabilize microtubules.

Properties of tubulin polymerization

Global summary of these properties:

• At concentrations of αβ-tubulin above Cc the dimers are polymerized to form microtubules; below Cc, microtubules are depolluted.
• The nucleotide, GTP or GDP, coupled with β-tubulin, makes the Cc for the assembly at the ends (+) and (-) of a microtubule different; by analogy with the filamentous actin assembly, the end (+) is defined as the preferred by the assembly.
• With higher concentrations of αβ-tubulin to Cc for polymerization, the dimers are added in greater quantity to the end (+).
• When the concentration of αβ-tubulin is higher than the Cc of the end (+) but lower than the Cc of the (-), it can give a growth in one direction by adding subunits to one end and dissociating subunits from the opposite end.

These characteristics lead to the existence of a dynamic instability of the microtubules, which consists in the fact that, in the same cell, some microtubules are depolymerizing (catastrophe) and others are elongating (rescue).

Motor Proteins

There are proteins that take advantage of ATP hydrolysis to generate mechanical energy and move substances along microtubules. These are dynein, the retrograde transporter, and kinesin, the anterograde transporter.

• Dinein is a structure molecule similar to kinesin: it consists of two identical heavy chains that make up two globular heads and a variable number of intermediate chains and light chains. They transport from the end (+) to the intramicrotubular channel (-). It is suggested that the ATP hydrolysis activity, the energy source of the cell, is found in the globular heads. Dinein transports gallbladders and oreganols, so it must interact with its membranes, and, to interact with them, it requires a protein complex, of which the dinactin is notable.

A kinesine attached to a microtubule.

• Most of the kines intervene in the atrograde transport of vesicles, that is, they imply a movement towards the most distal part of the cell or the neurita, from the end (-) to the (+) of the microtubules, on which they move. On the contrary, another family of motor proteins, dineins, employ the same rails but direct the vesicles to the most proximal part of the cell, so their transport is retrograde.

Pharmacology

There are a large number of drugs capable of binding to tubulin, modulating its activation state and thus interfering with microtubule dynamics at much lower intracellular concentrations than that of tubulin. In this way, cells stop their cell cycle and can lead to programmed cell death or apoptosis. Compounds that modulate tubulin activity can be broadly divided into two large groups: First, there are inhibitors of its polymerization, such as colchicine and vincristine, which bind to tubulin, preventing it from forming microtubules. On the other hand, there are microtubule stabilizing agents (MSAs), such as paclitaxel (commercially known as taxol) and docetaxel, which preferentially bind to assembled tubulin, minimizing dissociation of tubulin-GDP from the ends. microtubules and inducing the assembly of the normally inactive tubulin-GDP.

Drugs that modulate microtubule polymerization have been widely used in antitumor therapy. Being essential for mitosis and stopping it, it is possible to act against the tumor, but those tissues in rapid proliferation are also affected (bone marrow, intestinal mucosa...). The clinical success of paclitaxel and docetaxel has led to the search for new compounds with the same mechanism of action and to the discovery in recent years of a large number of microtubule stabilizing agents with at least two distinct binding sites.

Other functions

In addition to their structural role as a component of the cytoskeleton (together with actin and intermediate filaments), microtubules are involved in biological processes.

In development

The microtubule cytoskeleton is essential during the morphogenetic processes of development of organisms. For example, during embryogenesis in the fruit fly, Drosophila melanogaster, an intact and polarized microtubule network is required within the oocyte in order to establish the axes of the egg; In this way, the signals between the follicular cells and those of the oocyte (such as factors similar to TGF-alpha) cause the reorganization of the microtubules, placing their minus end in the anterior zone of the oocyte, which polarizes the structure and leads to the appearance of an anterior-dorsal axis. This involvement in body architecture also occurs in mammals.

Another field in which microtubules are essential is the formation of the nervous system in higher vertebrates; in them, the dynamics of tubulin and associated proteins (such as MAPs) are precisely controlled in order to develop the neuronal basis of the brain.

Regulation of Gene Expression

The cellular cytoskeleton is a dynamic element that acts at many levels in the cell: in addition to giving it a certain shape and structuring the traffic of vesicles and organelles, it can influence gene expression. However, the cellular pathways (that is, the signal transduction mechanisms) involved in this communication are poorly understood. However, the relationship between drug-mediated microtubule depolymerization and the specific expression of transcription factors and, therefore, the differential expression of genes dependent on the presence of these factors has been described. This communication between the cytoskeleton and regulation of the cellular response is also related to the generation of growth factors: for example, this relationship exists for connective tissue growth factor.

In cancer therapy, this fact is of vital importance since the antitumor drug paclitaxel targets the microtubule cytoskeleton, and it is precisely the interaction of the latter with elements that modulate the cell cycle that causes, in the presence of the antitumor, a series of cellular failures in cancer cells that lead to their programmed cell death, or apoptosis.