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Cytoskeleton of eukaryotes. Actin microfilaments are colored red, microtubules are colored green, cell nuclei are colored blue.

Cytoskeleton is a cellular framework or skeleton located in the cytoplasm of a living cell. It is present in all eukaryotic cells, and homologues of all eukaryotic cytoskeletal proteins are found in prokaryotic cells. The cytoskeleton is a dynamic, changing structure, the functions of which include maintaining and adapting the shape of the cell to external influences, exo- and endocytosis, ensuring the movement of the cell as a whole, active intracellular transport and cell division.

Keratin intermediate filaments in the cell.

The cytoskeleton is formed by proteins; several main systems are distinguished, named either by the main structural elements visible during electron microscopic studies (microfilaments, intermediate filaments, microtubules), or by the main proteins that make up them (actin-myosin system, keratins, tubulin - dynein system).

Cytoskeleton of eukaryotes

Actin filaments (microfilaments)

About 7 nm in diameter, microfilaments are two chains of actin monomers twisted into a spiral. They are mainly concentrated near the outer membrane of the cell, since they are responsible for the shape of the cell and are capable of forming protrusions on the cell surface (pseudopodia and microvilli). They are also involved in intercellular interaction (formation of adhesive contacts), signal transmission and, together with myosin, in muscle contraction. With the help of cytoplasmic myosins, vesicular transport can be carried out along microfilaments.

Intermediate filaments

Cytoskeleton of prokaryotes

For a long time, it was believed that only eukaryotes possess a cytoskeleton. However, with the publication of the 2001 article by Jones et al. (PMID 11290328), describing the role of bacterial actin homologues in cells Bacillus subtilis, a period of active study of the elements of the bacterial cytoskeleton began. To date, bacterial homologues of all three types of eukaryotic cytoskeletal elements - tubulin, actin and intermediate filaments - have been found. It has also been established that at least one group of bacterial cytoskeletal proteins, MinD/ParA, has no eukaryotic counterparts.

Bacterial homologs of actin

The most studied actin-like cytoskeletal components include MreB, ParM and MamK.

MreB and its homologues

MreB proteins and its homologues are actin-like components of the bacterial cytoskeleton that play an important role in maintaining cell shape, chromosome segregation, and organization of membrane structures. Some types of bacteria such as Escherichia coli, have only one MreB protein, while others may have 2 or more MreB-like proteins. An example of the latter is the bacterium Bacillus subtilis, in which the proteins MreB, Mbl ( M re B-l ike) and MreBH ( MreB h omolog).

In genomes E. coli And B. subtilis the gene responsible for the synthesis of MreB is located in the same operon with the genes for the MreC and MreD proteins. Mutations that suppress the expression of this operon lead to the formation of spherical cells with reduced viability.

Subunits of the MreB protein form filaments that wrap around the rod-shaped bacterial cell. They are located on the inner surface of the cytoplasmic membrane. The filaments formed by MreB are dynamic, constantly undergoing polymerization and depolymerization. Immediately before cell division, MreB is concentrated in the region in which the constriction will form. It is believed that MreB also functions to coordinate the synthesis of murein, a cell wall polymer.

Genes responsible for the synthesis of MreB homologues were found only in rod-shaped bacteria and were not found in cocci.

ParM

The ParM protein is present in cells containing low-copy plasmids. Its function is to propagate plasmids to the cell poles. In this case, the protein subunits form filaments elongated along the major axis of the rod-shaped cell.

The structure of the filament is a double helix. The growth of filaments formed by ParM is possible from both ends, in contrast to actin filaments, which grow only at the ± pole.

MamK

MamK is an actin-like protein Magnetospirillum magneticum, responsible for the correct location of magnetosomes. Magnetosomes are invaginations of the cytoplasmic membrane surrounding iron particles. The MamK filament acts as a guide along which magnetosomes are located, one after another. In the absence of the MamK protein, magnetosomes are distributed randomly over the cell surface.

-A set of thread-like protein structures - microtubules and microfilaments that make up the musculoskeletal system of the cell.

The cytoskeleton is a highly dynamic cytoplasmic system. Many cytoskeletal structures can easily be destroyed and reappear, changing their location or morphology. These cytoskeletal features are based on polymerization-depolymerization reactions of the main structural cytoskeletal proteins and their interaction with other proteins, both structural and regulatory.

Only eukaryotic cells have a cytoskeleton; prokaryotic (bacterial) cells do not have it, which is an important difference between these two types of cells. The cytoskeleton gives the cell a certain shape even in the absence of a rigid cell wall. It organizes the movement of organelles in the cytoplasm (the so-called flow of protoplasm), which underlies amoeboid movement. The cytoskeleton is easily rebuilt, providing, if necessary, a change in cell shape. The ability of cells to change shape determines the movement of cell layers in the early stages of embryonic development. During cell division (mitosis), the cytoskeleton “disassembles” (dissociates), and in daughter cells its self-assembly occurs again.

The functions of the cytoskeleton are diverse. It helps maintain cell shape and carries out all types of cellular movements. In addition, the cytoskeleton can take part in the regulation of the metabolic activity of the cell.

The cytoskeleton is formed by proteins. In the cytoskeleton, several main systems are distinguished, named either by the main structural elements visible during electron microscopic studies (Microfilaments, intermediate filaments, microtubules), or by the main proteins included in their composition (actin-myosin system, keratins, tubulin-dynein system ).

Intermediate filaments are the least understood structure among the major components of the cytoskeleton with respect to their assembly, dynamics, and function. Their properties and dynamics are very different from those of both microtubules and actin filaments. The functions of intermediate filaments still remain in the realm of hypotheses.

Cytoplasmic intermediate filaments are found in the vast majority of ukaryotic cells, both in vertebrates and invertebrates, and in higher plants. Rare examples of animal cells in which intermediate filaments are not found cannot be considered definitive, since intermediate filament proteins can form unusual structures.

Morphological microtubules are hollow cylinders with a diameter of about 25 nm with a wall thickness of about 5 nm. The cylinder wall consists of protofilaments - linear tubulin polymers with longitudinally oriented heterodimers. As part of microtubules, protofilaments run along their long axis with a slight shift relative to each other, so that the tubulin subunits form a three-start helix. The microtubules of most animals contain 13 protofilaments.

Actin filaments play a key role in the contractile apparatus of muscle and non-muscle cells, and also take part in many other cellular processes, such as motility, maintaining cell shape, cytokinesis

Actin filaments or fibrillar actin (F-actin) are thin fibrils with a diameter of 6-8 nm. They are the result of polymerization of globular actin - G-actin. In a cell, actin filaments, with the help of other proteins, can form many different structures.

Plan:

Introduction

• 1 Cytoskeleton of eukaryotes
  • 1.1 Actin filaments (microfilaments)
  • 1.2 Intermediate filaments
  • 1.3 Microtubules
• 2 Cytoskeleton of prokaryotes
  • 2.1 Bacterial homologs of actin
    • 2.1.1 MreB and its homologues
    • 2.1.2 ParM
    • 2.1.3 MamK
  • 2.2 Tubulin homologues
    • 2.2.1 FtsZ
    • 2.2.2 BtubA/B
  • 2.3 Crescentin, a homolog of intermediate filament proteins
  • 2.4 MinD and ParA
Notes

Introduction

Cytoskeleton of eukaryotes. Actin microfilaments are colored red, microtubules are colored green, cell nuclei are colored blue.

Cytoskeleton is a cellular framework or skeleton located in the cytoplasm of a living cell. It is present in all cells of both eukaryotes and prokaryotes. This is a dynamic, changing structure, the functions of which include maintaining and adapting the shape of the cell to external influences, exo- and endocytosis, ensuring the movement of the cell as a whole, active intracellular transport and cell division.

Keratin intermediate filaments in the cell.

The cytoskeleton is formed by proteins. In the cytoskeleton, several main systems are distinguished, named either by the main structural elements visible during electron microscopic studies (microfilaments, intermediate filaments, microtubules), or by the main proteins included in their composition (actin-myosin system, keratins, tubulin-dynein system ).

1. Cytoskeleton of eukaryotes

Eukaryotic cells contain three types of so-called filaments. These are supramolecular, extended structures consisting of proteins of the same type, similar to polymers. The difference is that in polymers the connection between monomers is covalent, but in filaments the connection between the constituent units is ensured due to weak non-covalent interaction.

1.1. Actin filaments (microfilaments)

About 7 nm in diameter, microfilaments are two chains of actin monomers twisted into a spiral. They are mainly concentrated near the outer membrane of the cell, since they are responsible for the shape of the cell and are capable of forming protrusions on the cell surface (pseudopodia and microvilli). They are also involved in intercellular interaction (formation of adhesive contacts), signal transmission and, together with myosin, in muscle contraction. With the help of cytoplasmic myosins, vesicular transport can be carried out along microfilaments.

1.2. Intermediate filaments

The diameter of intermediate filaments ranges from 8 to 11 nanometers. They consist of various kinds of subunits and are the least dynamic part of the cytoskeleton.

Diagram showing the cytoplasm, along with its components (or organelles), in a typical animal cell. Organelles:
(1) Nucleolus
(2) Core
(3) ribosome (small dots)
(4) Vesicle
(5) rough endoplasmic reticulum (ER)
(6) Golgi apparatus
(7) Cytoskeleton
(8) Smooth endoplasmic reticulum
(9) Mitochondria
(10) Vacuole
(11) Cytoplasm
(12) Lysosome
(13) Centriole and Centrosome

1.3. Microtubules

Microtubules are hollow cylinders about 25 nm in diameter, the walls of which are composed of 13 protofilaments, each of which is a linear polymer of a tubulin protein dimer. The dimer consists of two subunits - the alpha and beta forms of tubulin. Microtubules are extremely dynamic structures that consume GTP during polymerization. They play a key role in intracellular transport (serve as “rails” along which molecular motors - kinesin and dynein move), form the basis of the undilipodium axoneme and the spindle during mitosis and meiosis.

2. Cytoskeleton of prokaryotes

For a long time it was believed that only eukaryotes have a cytoskeleton. However, with the publication of the 2001 article by Jones et al. (PMID: 11290328), describing the role of bacterial actin homologues in cells Bacillus subtilis, a period of active study of the elements of the bacterial cytoskeleton began. To date, bacterial homologues of all three types of eukaryotic cytoskeletal elements have been found - tubulin, actin and intermediate filaments. It has also been established that at least one group of bacterial cytoskeletal proteins, MinD/ParA, has no eukaryotic counterparts.

2.1. Bacterial homologs of actin

The most studied actin-like cytoskeletal components include MreB, ParM and MamK.

2.1.1. MreB and its homologues

MreB proteins and its homologues are actin-like components of the bacterial cytoskeleton that play an important role in maintaining cell shape, chromosome segregation, and organization of membrane structures. Some types of bacteria such as Escherichia coli, have only one MreB protein, while others may have 2 or more MreB-like proteins. An example of the latter is the bacterium Bacillus subtilis, in which the proteins MreB, Mbl ( M re B-l ike) and MreBH ( MreB h omolog).

In genomes E. coli And B. subtilis the gene responsible for the synthesis of MreB is located in the same operon with the genes for the MreC and MreD proteins. Mutations that suppress the expression of this operon lead to the formation of spherical cells with reduced viability.

Subunits of the MreB protein form filaments that wrap around the rod-shaped bacterial cell. They are located on the inner surface of the cytoplasmic membrane. The filaments formed by MreB are dynamic, constantly undergoing polymerization and depolymerization. Immediately before cell division, MreB is concentrated in the region in which the constriction will form. It is believed that MreB also functions to coordinate the synthesis of murein, a cell wall polymer.

Genes responsible for the synthesis of MreB homologues were found only in rod-shaped bacteria and were not found in cocci.

2.1.2. ParM

The ParM protein is present in cells containing low-copy plasmids. Its function is to propagate plasmids to the cell poles. In this case, the protein subunits form filaments elongated along the major axis of the rod-shaped cell.

The structure of the filament is a double helix. The growth of filaments formed by ParM is possible from both ends, in contrast to actin filaments, which grow only at the ± pole.

2.1.3. MamK

MamK is an actin-like protein Magnetospirillum magneticum, responsible for the correct location of magnetosomes. Magnetosomes are invaginations of the cytoplasmic membrane surrounding iron particles. The MamK filament acts as a guide along which magnetosomes are located, one after another. In the absence of the MamK protein, magnetosomes are distributed randomly over the cell surface.

2.2. Tubulin homologues

Currently, two tubulin homologs have been found in prokaryotes: FtsZ and BtubA/B. Like eukaryotic tubulin, these proteins have GTPase activity.

2.2.1. FtsZ

The FtsZ protein is extremely important for bacterial cell division; it is found in almost all eubacteria and archaea. Also, homologs of this protein were found in eukaryotic plastids, which is another confirmation of their symbiotic origin.

FtsZ forms a so-called Z-ring, which acts as a scaffold for additional cell division proteins. Together they represent the structure responsible for the formation of the constriction (septum).

2.2.2. BtubA/B

Unlike the widespread FtsZ, these proteins are found only in bacteria of the genus Prosthecobacter. They are closer in structure to tubulin than FtsZ.

2.3. Crescentin, a homolog of intermediate filament proteins

The protein was found in cells Caulobacter crescentus. Its function is to give cells C. crescentus vibrio forms. In the absence of expression of the crescentin gene, cells C. crescentus take on the shape of a stick. Interestingly, the cells of the double mutants, crescentin − and MreB − , have a spherical shape.

2.4. MinD and ParA

These proteins have no homologs among eukaryotes.

MinD is responsible for the position of the division site in bacteria and plastids. ParA is involved in the partitioning of DNA into daughter cells.

Notes

1. Shih Y.-L., Rothfield L. The Bacterial Cytoskeleton. // Microbiology And Molecular Biology Reviews. - 2006. - V. 70., No. 3 - pp. 729-754. PMID: 16959967 - www.ncbi.nlm.nih.gov/sites/entrez?cmd=Retrieve&db=pubmed&dopt=AbstractPlus&list_uids=16959967

Key proteins of the cytoskeleton of eukaryotes, but also proteins that have no analogues in eukaryotes. Cytoskeletal elements play important roles in cell division, defense, shape maintenance, and polarity determination in various prokaryotes.

The first described cytoskeletal element of prokaryotes, it forms a ring structure in the middle of the cell, known as the Z-ring, which contracts during cell division, similar to the actin-myosin contractile ring of eukaryotes. The Z-ring is a highly dynamic structure consisting of numerous bundles of protofilaments, and the mechanisms of Z-ring compression, as well as the number of protofilaments, remain unknown. FtsZ functions as an organizing protein and is required for cell division, recruiting all known division-essential proteins to the division site.

Despite its functional proximity to actin, FtsZ is homologous to the eukaryotic microtubule-forming protein tubulin. Although comparison of the primary structures (i.e., amino acid sequences) of FtsZ and tubulin indicates only loose similarity, their three-dimensional structures are remarkably similar. Moreover, like tubulin, monomeric FtsZ is associated with GTP, and its polymerization with other FtsZ monomers occurs with the expenditure of GTP energy, similar to the dimerization of tubulin. Because FtsZ is required for bacterial cell division, it may serve as a target for antibiotics.

The mechanism of divergence of plasmid copies, which occurs with the participation of ParM filaments

ParM is a cytoskeletal element that is structurally similar to actin, but functions as tubulin. In addition, it polymerizes bidirectionally and exhibits dynamic instability, as is characteristic of tubulin polymerization. It forms a system with ParR and parC, which is necessary for the separation of R1 plasmids. ParM is attached to ParR -, which specifically binds to 10 direct repeats in the region parC R1 plasmids. ParM attaches to ParR at the two ends of its filament. Next, the filament lengthens, pulling two copies of plasmid R1 in different directions. The operation of this system is similar to the separation of chromosomes during eukaryotic cell division, and ParM functions like tubulin in the spindle, ParR functions like a kinetochore, and parC- as a centromere of a chromosome. The separation of F plasmids occurs in a similar way: the SopA protein functions as a cytoskeletal filament, and the SopB protein binds to the region sopC F-plasmids, like kinetochores and centromeres, respectively. An actin-like homolog of ParM was also found in a gram-positive bacterium Bacillus thuringiensis. It assembles into microtubule-like structures and is involved in the separation of replicated plasmids.

The MinCDE system is a filament system that places the septum strictly in the middle of the cell Escherichia coli. MinC prevents septum formation by interfering with FtsZ polymerization. MinC, MinD and MinE form a helical structure that wraps around the cell and is connected to the inside of the membrane by the MinD protein. The MinCDE helix occupies the poles and terminates a filamentous structure known as the E-ring, composed of the MinE protein and located in the middle part of the polar region. The E-ring contracts as it approaches the pole and disassembles the MinCDE helix as it moves. In this case, the separated components of the E-ring are assembled at the opposite pole and the disassembly of the MinCDE helix begins from the other end. The process is repeated, and the MinCDE helix oscillates between positions at the two poles of the cell. This oscillation continues throughout the cell cycle, causing the concentration of the protein MinC, which inhibits septum formation, to be lower in the middle of the cell than at the poles. The dynamic behavior of Min proteins has been reconstructed in vitro, where the artificial lipid bilayer acted as an analogue of the membrane.

Bactofilin is a cytoskeletal protein that forms filaments throughout the cell

Introduction

The very concept of the cytoskeleton or skeletal components of the cytoplasm of different cells was expressed by N.K. Koltsov, an outstanding Russian cytologist at the beginning of the 20th century. Unfortunately, they were forgotten and only in the late 1950s, with the help of an electron microscope, this skeletal system was rediscovered.

A huge contribution to the study of the cytoskeleton was made by the method of immunofluorescence, which helped to understand the chemistry and dynamics of this extremely important component of the cell. Cytoskeletal components are represented by thread-like, non-branching protein complexes, or filaments (thin threads).

There are three filament systems that differ in chemical composition, ultrastructure and functional properties. The thinnest threads are microfilaments; their diameter is about 6 nm and they consist mainly of the protein actin. Another group of filamentous structures includes microtubules, which have a diameter of 25 nm and consist mainly of the protein tubulin. The third group is represented by intermediate filaments with a diameter of about 10 nm (intermediate compared to 6 and 25 nm), formed from different but related proteins.

Chapter 1. Cytoskeleton

The cytoskeleton is a cellular framework or skeleton located in the cytoplasm of a living cell. It is present in all cells, both eukaryotes and prokaryotes. This is a dynamic, changing structure, the functions of which include maintaining and adapting the shape of the cell to external influences, exo- and endocytosis, ensuring the movement of the cell as a whole, active intracellular transport and cell division.

Cytoskeleton of eukaryotes

Eukaryotic cells contain three types of so-called filaments. These are supramolecular, extended structures consisting of proteins of the same type, similar to polymers. The difference is that in polymers the connection between monomers is covalent, but in filaments the connection between the constituent units is ensured due to weak non-covalent interaction.

Actin filaments (microfilaments)

About 7 nm in diameter, microfilaments are two chains of actin monomers twisted into a spiral. They are mainly concentrated near the outer membrane of the cell, since they are responsible for the shape of the cell and are capable of forming protrusions on the cell surface (pseudopodia and microvilli). They are also involved in intercellular interaction (formation of adhesive contacts), signal transmission and, together with myosin, in muscle contraction. With the help of cytoplasmic myosins, vesicular transport can be carried out along microfilaments.

Intermediate filaments

The diameter of intermediate filaments ranges from 8 to 11 nanometers. They consist of various kinds of subunits and are the least dynamic part of the cytoskeleton.

Microtubules

Microtubules are hollow cylinders about 25 nm in diameter, the walls of which are composed of 13 protofilaments, each of which is a linear polymer of a tubulin protein dimer. The dimer consists of two subunits - the alpha and beta forms of tubulin. Microtubules are extremely dynamic structures that consume GTP during polymerization. They play a key role in intracellular transport (serve as “rails” along which molecular motors - kinesin and dynein move), form the basis of the undilipodium axoneme and the spindle during mitosis and meiosis.

Cytoskeleton of prokaryotes

For a long time it was believed that only eukaryotes have a cytoskeleton. However, with the publication in 2001 of an article by Jones et al., describing the role of bacterial homologs of actin in Bacillus subtilis cells, a period of active study of the elements of the bacterial cytoskeleton began. To date, bacterial homologues of all three types of eukaryotic cytoskeletal elements - tubulin, actin and intermediate filaments - have been found. It has also been established that at least one group of bacterial cytoskeletal proteins, MinD/ParA, has no eukaryotic counterparts.

The cytoskeleton is formed by proteins. In the cytoskeleton, several main systems are distinguished, named either by the main structural elements visible during electron microscopic studies (microfilaments, intermediate filaments, microtubules), or by the main proteins included in their composition (actin-myosin system, keratins, tubulin-dynein system ).

Bacterial homologs of actin

MreB and its homologues

MreB proteins and its homologues are actin-like components of the bacterial cytoskeleton that play an important role in maintaining cell shape, chromosome segregation, and organization of membrane structures. Some bacterial species, such as Escherichia coli, have only one MreB protein, while others may have 2 or more MreB-like proteins. An example of the latter is the bacterium Bacillus subtilis, in which the proteins MreB, Mbl (MreB-like) and MreBH (MreB homolog) were discovered.

In the genomes of E. coli and B. subtilis, the gene responsible for the synthesis of MreB is located in the same operon with the genes for the MreC and MreD proteins. Mutations that suppress the expression of this operon lead to the formation of spherical cells with reduced viability.

Subunits of the MreB protein form filaments that wrap around the rod-shaped bacterial cell. They are located on the inner surface of the cytoplasmic membrane. The filaments formed by MreB are dynamic, constantly undergoing polymerization and depolymerization. Immediately before cell division, MreB is concentrated in the region in which the constriction will form. It is believed that MreB also functions to coordinate the synthesis of murein, a cell wall polymer.

Genes responsible for the synthesis of MreB homologues were found only in rod-shaped bacteria and were not found in cocci.

The ParM protein is present in cells containing low-copy plasmids. Its function is to propagate plasmids to the cell poles. In this case, the protein subunits form filaments elongated along the major axis of the rod-shaped cell.

The structure of the filament is a double helix. The growth of filaments formed by ParM is possible from both ends, in contrast to actin filaments, which grow only at the ± pole.

MamK is an actin-like protein from Magnetospirillum magneticum that is responsible for the correct positioning of magnetosomes. Magnetosomes are invaginations of the cytoplasmic membrane surrounding iron particles. The MamK filament acts as a guide along which magnetosomes are located, one after another. In the absence of the MamK protein, magnetosomes are distributed randomly over the cell surface.

Tubulin homologues

Currently, two tubulin homologs have been found in prokaryotes: FtsZ and BtubA/B. Like eukaryotic tubulin, these proteins have GTPase activity.

The FtsZ protein is extremely important for bacterial cell division; it is found in almost all eubacteria and archaea. Also, homologs of this protein were found in eukaryotic plastids, which is another confirmation of their symbiotic origin.

FtsZ forms a so-called Z-ring, which acts as a scaffold for additional cell division proteins. Together they represent the structure responsible for the formation of the constriction (septum).

Unlike the widespread FtsZ, these proteins are found only in bacteria of the genus Prosthecobacter. They are closer in structure to tubulin than FtsZ.

Crescentin

Crescentin, a homolog of intermediate filament proteins

The protein was found in Caulobacter crescentus cells. Its function is to give the cells of C. crescentus the form of a vibrio. In the absence of crescentin gene expression, C. crescentus cells take on a rod shape. Interestingly, the cells of the double mutants, crescentin− and MreB−, have a spherical shape.

MinD and ParA

These proteins have no homologs among eukaryotes.

MinD is responsible for the position of the division site in bacteria and plastids. ParA is involved in the partitioning of DNA into daughter cells.

Bacterial homologs of actin.

The most studied actin-like cytoskeletal components include MreB, ParM and MamK.

Chapter 2. Microtubules

Microtubules are protein intracellular structures that are part of the cytoskeleton.

Microtubules are hollow cylinders with a diameter of 25 nm. Their length can range from several micrometers to probably several millimeters in the axons of nerve cells. Their wall is formed by tubulin dimers. Microtubules, like actin microfilaments, are polar: microtubule self-assembly occurs at one end, and disassembly occurs at the other. In cells, microtubules serve as structural components and are involved in many cellular processes, including mitosis, cytokinesis, and vesicular transport.

Structure

Microtubules are structures in which 13 protofilaments, consisting of heterodimers of α- and β-tubulin, are arranged around the circumference of a hollow cylinder. The outer diameter of the cylinder is about 25 nm, the inner diameter is about 15.

One end of the microtubule, called the plus end, permanently attaches free tubulin to itself. From the opposite end - the minus end - tubulin units are split off.

There are three phases in microtubule formation:

1. Delayed phase, or nucleation. This is the stage of microtubule nucleation, when tubulin molecules begin to combine into larger formations. This connection occurs more slowly than the addition of tubulin to an already assembled microtubule, which is why the phase is called slow;

2. Polymerization phase, or elongation. If the concentration of free tubulin is high, its polymerization occurs faster than depolymerization at the minus end, causing the microtubule to elongate. As it grows, the tubulin concentration drops to a critical level and the growth rate slows down until entering the next phase;

3. Steady state phase. Depolymerization balances polymerization and microtubule growth stops.

Laboratory studies show that microtubule assembly from tubulins occurs only in the presence of guanosine triphosphate and magnesium ions.

Related information.