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Microtubule plays an imperative role in segregation of chromosome into daughter cells in meiosis. In meiosis process, microtubule otherwise known as meiotic spindle, extends from the cellular periphery organelle such as centrosome towards the centre of the cell. This growth of meiotic spindle from centrosome is made possible by Gamma-tubulin. Tubulin is the subunit of microtubule. Gamma-tubulin is a subtype of tubullin and functions in microtubule assembly. In addition, there are two more members of the tubulin family, such as Alpha-tubulin and Beta-tubulin. These two subtypes forms a dimer structure of tubulin, which are bound tightly together by non-covalent bonding. As a consequence, tubulin dimers stack on top of each other by non-covalent bonding, this ultimately forms the structure of microtubule which consists of 13 tubulin protofilaments. However, neither Alpha-tubulin nor Beta-tubulin has made contribution to microtubule nucleation, which directly affects the outgrowth of microtubule, hence meiotic spindle. This important function is solely filled by Gamma-Tubulin and associated proteins, collectively known as the Gamma-tubulin ring complex. This page will be primarily focusing on the characteristics of Gamma-tubulin and its role in meiosis. As a result, it is organised in the following orders. Firstly, history of Gamma-tubulin, followed by structure and function of Gamma-tubulin. Then, a discussion of Gamma-tubulin's involvement in meiosis. Fourthly, current project on Gamma-tubulin. Lastly, a conclusion of Gamma-tubulin.
Cell Biology Images Biotinylated_tubulin.jpeg 1986 Schulze and Kirschner - Microtubules turn over rapidly, chemically labeling microtubules to define their dynamics. JCB Archive Schulze, E., and M. Kirschner. 1986. J. Cell Biol. 102:1020–1031 1986 Figure. Tubulin
History of Gamma-tubulin
Gamma-tubulin was identified in 1989 as the third protein of the tubulin family which is located at the centrosomes. Experimental evidence suggests that Gamma-tubulin contributes largely in microtubule organisation in the centrosome. Gamma-tubulin was initially discovered in Aspergillus nidulans as the product of the mipA gene. Gamma-tubulin share approximately 30% amino acid identify with either Alpha-tubulin or Beta-tubulin, and Gamma-tubuiln appears to account for approximately 1% of the total tubulin pool in Xenopus. As a result of this, Gamma-tubuin was conserved in a number of species such as Schizosaccharomyces pombe, Xenopus laevis, Homo sapiens and Drosophila melanogaster, as well as in malaria parasite Plasmodium falciparum.
Structure of Gamma-tubulin
Gamma-tubulin forms a 25 nm diameter Gamma-tubulin ring complex
Homo Sapiens(Human) Gamma-tubulin subtypes include:
- TUBG1-Tubulin, Gamma 1
- TUBG2-Tubulin, Gamma 2
- TUBGCP2-Tubulin, gamma complex associated protein 2
- TUBGCP3-Tubulin, gamma complex associated protein 3
- TUBGCP4-Tubulin, gamma complex associated protein 4
- TUBGCP5-Tubulin, gamma complex associated protein 5
- TUBGCP6-Tubulin, gamma complex associated protein 6
Function of Gamma-tubulin
Gamma-tubulin functions significantly as an organising centre of microtubule assembly in eukaryotic cells. Gamma-tubulin accomplishes this role through the formation of Gamma-tubulin ring complex. This complex contains a number of copies of Gamma-tubulin as well as a number of other proteins that are involved in nucleation of microtubule. Microtubule, or in the case of meiosis such as meiotic spindle, are formed by outgrowth from centrosome. Centrosomes contain hundreds of ring-shaped structures formed by Gamma-tubulin, and each of the tubulin performs as the nucleation site for the growth of one microtubule. In addition, the outgrowth of microtubule adapts orientation specificity. That is, each Alpha-Beta tubulin dimers add to the Gamma-tubulin ring in a pre-mediated direction. As a consequence, the minus/Alpha end of each microtubule is embedded in the centrosome, and growth occurs at the plus/Beta end.
The Gamma-tubulin ring complex is required for microtubule nucleation. It achieves this goal through the so called 'Template' model. In template model, the Gamma-tubulin ring complex is overwhelmed with the minus/sAlpha end of the microtubule, as a result, it acts as a template. Furthermore, each Gamma-tubulin ring complex consists of 13 Gamma-tubulin molecules, which interact laterally with one another and each sit at the base of one protofilament.
Why do microtubules/meiotic spindles growth require Gamma-tubulin based nucleating sites?
Microtubules/meiotic spindles require nucleating sites such as those provided by the Gamma-tubulin in the centromsome, mainly because it is difficult to start a new microtubule. This requires the assembly of a ring of Alpha-Beta tubulin dimers, than to add such dimers to a preexisting microtubule structure. Purified free Alpha-Beta tubulin can polymerise spontaneously in vitro when at a high concentration, but in living cell, the concentration of free Alpha-Beta tubulin is too low to drive the difficult first step of assembling the initial ring of a new microtubule. Hence, by providing organising centres containing nucleation sites, and keeping the concentration of free Alpha-Beta tubulin dimers low, eukaryotic cells can control where microtubules form.
Involvement of Gamma-tubulin in Meiosis
Gamma-tubulin plays a crucial role in meiosis. It achieves this by acting as a meiotic spindle organising centre and providing nucleation sites. Meiotic spindles formed in meiosis 1, are responsible for separating the components of original cell into daughter cells. In prophase 1, meiotic spindles were being rapidly assembled through nucleation sites. In anaphase, meiotic spindles pull the chromosome materials towards the opposite ends of spindle pole, and making sure that each daughter cell receives the same amount of genetic information. Without Gamma-tubulin and associated protein components, meiotic spindle would not be formed, hence nuclear separation would not occur.
Current development of Gamma-tubulin
Gamma-tubulin is currently involved in the development of mouse fertilisation. The study was involved acentriolar mouse oocytes and their early development following fertilization. The aim of this study was to demonstrate Gamma-tubulin antibody crossreacts with a 50, 000 Mr protein in unfertilised mouse oocytes and demonstrate that Gamma-tubulin distribution is rearranged during fertilization.
In addition, Gamma-tubulin is involved in the development in Arabidopsis. This study aims to provide genetic evidence that in Arabidopsis thaliana, Gamm-tubulin is required for the formation of spindle, phargmoplast and cortical microtubule arrays.
- Alpha-tubulin: a subtype of tubulin with a negative polarity
- Anaphase 1: chromosomes at the middle of the cell are being pulled by spindles toward each daughter cells
- Beta-tubulin: a subtype of tubulin with a positive polarity
- Centrosome: centrally located organelle of animal cells that is the primary microtubule-organising centre and acts as the spindle pole during mitosis. In most animal cells it contains a pair of centrioles
- Gamma-tubulin: a third subtype of tubulin, and was identified as the microtubule assembly organising centre
- Gamma-tubulin ring complex: contains Gamma-tubulina and a number of associated proteins forms on centrosome
- Meiosis: special type of cell division by which eggs and sperm cells are made. Two successive nuclear divisions with only one round of DNA replication generates four haploid daughter cells from an initial diploid cell
- Microtubule: long, stiff, cylindrical structure composed of the protein tubulin. Used by eukaryotic cells to regulate their shape and control their movements
- Meiotic spindle: formed from the centrosome of eukaryotic cells
- Prophase 1: condensation of chromosomes
- Segregation: separating genetic materials in cells
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