The Role of the RanGTPase in Mitotic Spindle Assembly

Abstract

Mitosis is the process whereby a cell separates its paired chromosomes into two new identical daughter cells. To achieve this and maintain the correct amount of deoxyribo nuclic acid (DNA) in the cells, the cell forms a framework called the mitotic spindle onto which the chromosomes attach and then get pulled into the new daughter cells. The mitotic spindle is made up of microtubules and associated proteins. The GTPase Ran, which is important for nuclear import and export, plays important roles in regulating the formation of the mitotic spindle. Ran can bind and hydrolyse guanosine triphosphate (GTP), which enables it to regulate several cellular processes. Generation of Ran‐GTP at the chromosomes creates an intracellular gradient that provides directionality to nuclear cytoplasmic transport during interphase and creates an environment around the chromosomes to facilitate spindle assembly.

Key Concepts:

  • Ran is a small GTPase.

  • During interphase, spindle assembly factors bind to Importin‐β and are transported into the nucleus.

  • Ran‐GTP is generated at chromosomes and releases spindle assembly factors from inhibitory binding to Importin‐β.

  • Gradients of Ran‐GTP and released cargo proteins are generated in mitotic cells that are important for regulating many activities.

Keywords: Ran; GTPase; mitosis; mitotic spindle; importin; exportin

Figure 1.

Cartoon depicting the basic organisation of a mitotic spindle. Microtubules are polar structures with their ‘minus’ ends (−) at the spindle pole close to the centrosome and their ‘plus’ ends (+) toward the center of the spindle. Microtubules bundles that connect the spindle pole to the kinetochore are called k‐fibres.

Figure 2.

(a) Cartoon of the GTPase cycle of Ran. (b) Cartoon of depicting the role of the RanGTP cycle in nuclear transport. (1) Proteins to be imported into the nucleus possess a nuclear import signal (NLS) to which binds the nuclear import machinery, Importin α and β (blue). (2) Once in the nucleus RanGTP binds to Importin β and the complex dissociates. The RanGTP concentration is much higher in the nucleus because RCC1 a Ran nucleotide exchange factor (GEF) that generates RanGTP form RanGDP localises to the nucleus. (3) In the nucleus RanGTP can also bind to nuclear export factors (green), which then bind to proteins to be transported out o the nucleus. (4) On the cytosolic face of the nuclear pore the complex encounters RanGAP1 that stimulates the intrinsic GTPase activity of Ran converting RanGTP to RanGDP and cause the exported complex to dissociate.

Figure 3.

Cartoons representing the potential role of gradients in spindle assembly. The gradient of RanGTP (green) could acts as the key positioning factor during mitosis, where different factors are differentially regulated at different concentrations of RanGTP. Such a mechanism could position different spindle activities at different points of the gradient. A more complex version of this model suggests that the RanGTP gradient is a master gradient that may additionally act to establish additional biochemical gradients (e.g. by kinases depicted in red and blue). The combination of multiple gradients could establish very fine positioning information in similar ways described in many developing embryonic systems.

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Further Reading

Clarke PR and Zhang C (2008) Spatial and temporal coordination of mitosis by Ran GTPase. Nature Reviews in Molecular Cell Biology 9: 464–477.

Kalab P and Heald R (2008) The RanGTP gradient – a GPS for the mitotic spindle. Journal of Cell Science 121: 1577–1586.

Rush M and D'Eustachio (2001) The small GTPase Ran. Norwell, MA: Kluwer Academic Publishers.

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Renshaw, Matthew J, and Wilde, Andrew(May 2011) The Role of the RanGTPase in Mitotic Spindle Assembly. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0022522]