In
cell biology
Cell biology (also cellular biology or cytology) is a branch of biology that studies the structure, function, and behavior of cells. All living organisms are made of cells. A cell is the basic unit of life that is responsible for the living and ...
, microtubule nucleation is the event that initiates ''
de novo'' formation of
microtubules
Microtubules are polymers of tubulin that form part of the cytoskeleton and provide structure and shape to eukaryotic cells. Microtubules can be as long as 50 micrometres, as wide as 23 to 27 nm and have an inner diameter between 11 an ...
(MTs). These filaments of the
cytoskeleton
The cytoskeleton is a complex, dynamic network of interlinking protein filaments present in the cytoplasm of all cells, including those of bacteria and archaea. In eukaryotes, it extends from the cell nucleus to the cell membrane and is compos ...
typically form through
polymerization
In polymer chemistry, polymerization (American English), or polymerisation (British English), is a process of reacting monomer, monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. There are ...
of α- and β-
tubulin
Tubulin in molecular biology can refer either to the tubulin protein superfamily of globular proteins, or one of the member proteins of that superfamily. α- and β-tubulins polymerize into microtubules, a major component of the eukaryotic cytoske ...
dimers, the basic building blocks of the microtubule, which initially interact to nucleate a seed from which the filament elongates.
Microtubule nucleation occurs spontaneously ''
in vitro
''In vitro'' (meaning in glass, or ''in the glass'') studies are performed with microorganisms, cells, or biological molecules outside their normal biological context. Colloquially called "test-tube experiments", these studies in biology an ...
'', with solutions of purified tubulin giving rise to full-length polymers. The tubulin dimers that make up the polymers have an intrinsic capacity to self-aggregate and assemble into cylindrical tubes, provided there is an adequate supply of GTP. The kinetics barriers of such a process, however, mean that the rate at which microtubules spontaneously nucleate is relatively low.
Role of γ-tubulin and the γ-tubulin ring complex (γ-TuRC)
''
In vivo
Studies that are ''in vivo'' (Latin for "within the living"; often not italicized in English) are those in which the effects of various biological entities are tested on whole, living organisms or cells, usually animals, including humans, and ...
'', cells get around this kinetic barrier by using various proteins to aid microtubule nucleation. The primary pathway by which microtubule nucleation is assisted requires the action of a third type of tubulin,
γ-tubulin
Tubulin in molecular biology can refer either to the tubulin protein superfamily of globular proteins, or one of the member proteins of that superfamily. α- and β-tubulins polymerize into microtubules, a major component of the eukaryotic cytoske ...
, which is distinct from the α and β subunits that compose the microtubules themselves. The γ-tubulin combines with several other associated proteins to form a conical structure known as the γ-tubulin ring complex (γ-TuRC). This complex, with its 13-fold symmetry, acts as a scaffold or template for α/β tubulin dimers during the nucleation process—speeding up the assembly of the ring of 13
protofilaments that make up the growing microtubule. The γ-TuRC also acts as a cap of the (−) end while the microtubule continues growth from its (+) end. This cap provides both stability and protection to the microtubule (-) end from enzymes that could lead to its depolymerization, while also inhibiting (-) end growth.
MT Nucleation from Microtubule Organizing Centers (MTOCs)
The γ-TuRC is typically found as the core functional unit in a
microtubule organizing center The microtubule-organizing center (MTOC) is a structure found in eukaryotic cells from which microtubules emerge. MTOCs have two main functions: the organization of eukaryotic flagella and cilia and the organization of the mitotic and meiotic spind ...
(MTOC), such as the
centrosome
In cell biology, the centrosome (Latin centrum 'center' + Greek sōma 'body') (archaically cytocentre) is an organelle that serves as the main microtubule organizing center (MTOC) of the animal cell, as well as a regulator of cell-cycle progres ...
in
animal
Animals are multicellular, eukaryotic organisms in the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, are Motilit ...
cells or the
spindle pole bodies in
fungi
A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from ...
and
algae
Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular mic ...
. The γ-TuRCs in the centrosome nucleate an array of microtubules in
interphase
Interphase is the portion of the cell cycle that is not accompanied by visible changes under the microscope, and includes the G1, S and G2 phases. During interphase, the cell grows (G1), replicates its DNA (S) and prepares for mitosis (G2). A c ...
, which extend their (+)-ends radially outwards into the cytoplasm towards the periphery of the cell. Among its other functions, this radial array is used by microtubule-based
motor proteins
Motor proteins are a class of molecular motors that can move along the cytoplasm of cells. They convert chemical energy into mechanical work by the hydrolysis of ATP. Flagellar rotation, however, is powered by a proton pump.
Cellular functions ...
to transport various cargoes, such as vesicles, to the plasma membrane.
In animal cells undergoing
mitosis
In cell biology, mitosis () is a part of the cell cycle in which replicated chromosomes are separated into two new nuclei. Cell division by mitosis gives rise to genetically identical cells in which the total number of chromosomes is mainta ...
, a similar radial array is generated from two MTOCs called the
spindle pole
In cell biology, the spindle apparatus refers to the cytoskeletal structure of eukaryotic cells that forms during cell division to separate sister chromatids between daughter cells. It is referred to as the mitotic spindle during mitosis, a pr ...
s, which produce the bipolar mitotic spindle. Some cells however, such as those of higher
plant
Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclud ...
s and oocytes, lack distinct MTOCs and microtubules are nucleated via a non-centrosomal pathway. Other cells, such as neurons, skeletal muscle cells, and epithelial cells, which do have MTOCs, possess arrays of microtubules not associated with a centrosome. These non-centrosomal microtubule arrays can take on various geometries—such as those leading to the long, slender shape of
myotubes, the fine protrusions of an
axon
An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences), is a long, slender projection of a nerve cell, or neuron, in vertebrates, that typically conducts electrical impulses known as action po ...
, or the strongly polarized domains of an
epithelial cell
Epithelium or epithelial tissue is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed cells with a little intercellula ...
. Researchers think that the microtubules in these arrays are generated first by the γ-TuRCs, then transported via motor proteins or
treadmilling
In molecular biology, treadmilling is a phenomenon observed within protein filaments of the cytoskeletons of many cells, especially in actin filaments and microtubules. It occurs when one end of a filament grows in length while the other end shri ...
to their desired location, and finally stabilized in the needed configuration through the action of various anchoring and cross-linking proteins.
In the cortical array of plants, as well as in the axons of neurons, scientists believe that microtubules nucleate from existing microtubules via the action of severing enzymes such as
katanin Katanin is a microtubule-severing AAA protein. It is named after the Japanese sword called a katana. Katanin is a heterodimeric protein first discovered in sea urchins. It contains a 60 kDa ATPase subunit, encoded by ''KATNA1'', which functions ...
. Akin to the action of
cofilin
ADF/cofilin is a family of actin-binding proteins associated with the rapid depolymerization of actin microfilaments that give actin its characteristic dynamic instability. This dynamic instability is central to actin's role in muscle contractio ...
in generating actin filament arrays, the severing of microtubules by
MAPs
A map is a symbolic depiction emphasizing relationships between elements of some space, such as objects, regions, or themes.
Many maps are static, fixed to paper or some other durable medium, while others are dynamic or interactive. Although ...
creates new (+) ends from which microtubules can grow. In this fashion dynamic arrays of microtubules can be generated without the aid of the γ-TuRC.
Branching MT nucleation
Studies using Xenopus egg extracts have identified a novel form of microtubule nucleation that generates fan-like branching arrays, in which new microtubules grow at an angle off of older microtubules. These branching microtubules maintain the same polarity as their mother microtubules, and their assembly involves the binding of non-centrosomal γ-TuRCs to the sides of existing microtubules through the
augmin complex. This method of microtubule-dependent microtubule nucleation leads to rapid amplification in microtubule density.
Branching MT nucleation has been observed in numerous organisms both in the plant and animal kingdoms. Through use of
TIRF microscopy, researchers have visually observed the nucleation of branching microtubules in ''
Drosophila
''Drosophila'' () is a genus of flies, belonging to the family Drosophilidae, whose members are often called "small fruit flies" or (less frequently) pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species ...
'' cells during the formation of the mitotic spindle. Five proteins in ''
Drosophila
''Drosophila'' () is a genus of flies, belonging to the family Drosophilidae, whose members are often called "small fruit flies" or (less frequently) pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species ...
'' (DGT2 through DGT6) have been identified that are necessary and responsible for facilitating the localization of γ-tubulin to existing MTs and are not associated with its localization at the centrosome.
Role of Microtubule-Associated Proteins (MAPs)
Though the γ-TuRC is the primary protein used to nucleate microtubules, it is not the only protein that acts as a nucleation factor. Several other
MAPs
A map is a symbolic depiction emphasizing relationships between elements of some space, such as objects, regions, or themes.
Many maps are static, fixed to paper or some other durable medium, while others are dynamic or interactive. Although ...
assist the γ-TuRC with the nucleation process, while others nucleate microtubules independently of γ-TuRC. In the branching nucleation described above, the addition of
TPX2
Targeting protein for Xklp2 is a protein that in humans is encoded by the ''TPX2'' gene. It is one of the many spindle assembly factors that play a key role in inducing microtubule assembly and growth during M phase.
Key domains of TPX2
TPX2 has ...
to the egg extracts led to a dramatic increase in nucleation events—while in other studies, the protein
XMAP215
The XMAP215/Dis1 family is a highly conserved group of microtubule-associated proteins (MAPs) in eukaryotic organisms. These proteins are unique MAPs because they primarily interact with the growing-end (plus-end) of microtubules. This special pro ...
, ''in vitro'', nucleated microtubule asters with its depletion ''in vivo'' reducing nucleation potential of centrosomes. The microtubule-binding protein
doublecortin
Neuronal migration protein doublecortin, also known as doublin or lissencephalin-X is a protein that in humans is encoded by the DCX gene.
Function
Doublecortin (DCX) is a microtubule-associated protein expressed by neuronal precursor cells an ...
, ''in vitro'', nucleates microtubules—acting by binding to the side rather than the end of growing microtubules. Thus, there may be a family of nucleation factor proteins in cells that use a variety of mechanisms to lower the energetic cost of nucleating microtubules. Recent studies have provided evidence towards the concept that the promotion of microtubule nucleation is possible with a combination of α- and β-
tubulin
Tubulin in molecular biology can refer either to the tubulin protein superfamily of globular proteins, or one of the member proteins of that superfamily. α- and β-tubulins polymerize into microtubules, a major component of the eukaryotic cytoske ...
dimers and the aforementioned
MAP
A map is a symbolic depiction emphasizing relationships between elements of some space, such as objects, regions, or themes.
Many maps are static, fixed to paper or some other durable medium, while others are dynamic or interactive. Although ...
TPX2
Targeting protein for Xklp2 is a protein that in humans is encoded by the ''TPX2'' gene. It is one of the many spindle assembly factors that play a key role in inducing microtubule assembly and growth during M phase.
Key domains of TPX2
TPX2 has ...
, even in the absence of γ-TuRC.
Several proteins are involved in formatting the γ-TuRC and temporal and spatial control of microtubule nucleation. These include, for example,
coiled-coil
A coiled coil is a structural motif in proteins in which 2–7
alpha helix, alpha-helices are coiled together like the strands of a rope. (Protein dimer, Dimers and Protein trimer, trimers are the most common types.) Many coiled coil-type protei ...
proteins with structural functions and regulatory proteins, such as components of the
Ran cycle.
NEDD1
Neural precursor cell expressed, developmentally down-regulated 1, also known as Nedd1, is a human gene
and encodes the protein NEDD1.
NEDD1 is localized in the centrosome and it plays a role in mitosis through its interaction with γ-tubulin. W ...
recruits the γ-TuRC to the
centrosome
In cell biology, the centrosome (Latin centrum 'center' + Greek sōma 'body') (archaically cytocentre) is an organelle that serves as the main microtubule organizing center (MTOC) of the animal cell, as well as a regulator of cell-cycle progres ...
by binding to γ-tubulin.
References
External links
MBInfo: MicrotubuleMBInfo: Tubulin Complex Assembly
{{DEFAULTSORT:Microtubule Nucleation
Cell anatomy
Cytoskeleton