Microtubule

Microtubule

Microtubules are a component of the cytoskeleton. These rope-like polymers of tubulin can grow as long as 25 micrometers and are highly dynamic. The outer diameter of microtubule is about 25 nm. Microtubules are important for maintaining cell structure, providing platforms for intracellular...

s are filaments of the cytoskeleton

Cytoskeleton

The cytoskeleton is a cellular "scaffolding" or "skeleton" contained within a cell's cytoplasm and is made out of protein. The cytoskeleton is present in all cells; it was once thought to be unique to eukaryotes, but recent research has identified the prokaryotic cytoskeleton...

. They typically form through the polymerization

Polymerization

In polymer chemistry, polymerization is a process of reacting monomer molecules together in a chemical reaction to form three-dimensional networks or polymer chains...

 of α- and β-tubulin

Tubulin

Tubulin is one of several members of a small family of globular proteins. The most common members of the tubulin family are α-tubulin and β-tubulin, the proteins that make up microtubules. Each has a molecular weight of approximately 55 kiloDaltons. Microtubules are assembled from dimers of α- and...

 dimers elongating existing microtubules. The de novo

De novo synthesis

De novo synthesis refers to the synthesis of complex molecules from simple molecules such as sugars or amino acids, as opposed to their being recycled after partial degradation. For example, nucleotides are not needed in the diet as they can be constructed from small precursor molecules such as...

formation of microtubules requires an initiation event called microtubule nucleation.

Microtubule nucleation requires the action of a third type of tubulin, γ-tubulin, which is distinct from the α and β subunits which compose the microtubules themselves. The γ-tubulin combines with several other associated proteins to form a circular structure known as the γ-tubulin ring complex (γ-TuRC). This complex acts as a scaffold for α/β tubulin dimers to begin polymerization. It also acts as a cap of the (−) end while microtubule growth continues towards the (+) direction.

The γ-TuRC is typically found as the core functional unit in a microtubule organizing center

Microtubule organizing center

The microtubule-organizing center 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 spindle apparatus, which separate the chromosomes during cell...

 (MTOC), such as the centrosome

Centrosome

In cell biology, the centrosome is an organelle that serves as the main microtubule organizing center of the animal cell as well as a regulator of cell-cycle progression. It was discovered by Edouard Van Beneden in 1883...

 in animal

Animal

Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...

 cells or the spindle pole bodies

Spindle pole body

The spindle pole body is the microtubule organizing center in yeast cells, functionally equivalent to the centrosome. Unlike the centrosome the SPB does not contain centrioles. The SPB organises the microtubule cytoskeleton which plays many roles in the cell...

 in fungi

Fungus

A fungus is a member of a large 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, Fungi, which is separate from plants, animals, and bacteria...

 and algae

Algae

Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...

. However, the cells of higher plant

Plant

Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...

s for example lack distinct MTOCs and microtubules are nucleated at γ-TuRCs distributed around the nuclear envelope

Nuclear envelope

A nuclear envelope is a double lipid bilayer that encloses the genetic material in eukaryotic cells. The nuclear envelope also serves as the physical barrier, separating the contents of the nucleus from the cytosol...

. Microtubules can also nucleate in the cytoplasm

Cytoplasm

The cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...

, forming so-called astral microtubules radiating star-like from a diffusible nucleation center.

Several proteins are involved in the formation of the γ-TuRC and the temporal and spatial control of microtubule nucleation. These include for example coiled-coil proteins with structural functions and regulatory proteins, such as components of the Ran cycle

Ran (biology)

Ran is a small 25Kda protein that is involved in transport into and out of the cell nucleus during interphase and also involved in mitosis. It is a member of the Ras superfamily....

.

In some filamentous fungi gamma tubulin localizes at the tip.