Microtubules are a non-covalent helical polymer formed by the globular protein tubulin and act as "conveyer belts" inside the cells. They move vesicles, granules and organelles (mitochondria, chromosomes).
During in vitro microtubule assembly, tubulin heterodimers join end-to-end to form protofilaments (linear tubulin polymer rows), which associate laterally to form microtubules. GTP must be bound to both α and β subunits for a tubulin heterodimer to associate with other heterodimers.
When a tubulin molecule adds to the microtubule, the GTP is hydrolyzed to GDP. Microtubules will also form normally with nonhydrolyzable GTP analogs (like GpCpp [GMPCPP]) attached. However, in this case they will not be able to depolymerize. Thus, the normal role of GTP hydrolysis is to promote the constant growth of microtubules as they are needed by a cell.
In living cells, microtubules exist in an unusual dynamic equilibrium with tubulin subunits, which is called dynamic instability. Individual microtubules alternate between polymerization and de-polymerization periods, leading to rapid exchange between tubulin subunits and microtubule polymer. Dynamic instability responds to the needs of the cell in terms of microtubule (de)formation and distribution.
| Nucleotide analog | Properties |
|---|---|
| ITP | highly promotive |
| GTP | standard for reactivity of other analogs |
| GpCpp (GMPCPP) | moderately promotive, completely suppresses dynamic instability - microtubules do not depolymerize |
| caged-GpCpp (caged-GMPCPP) | "caged" derivative of GpCpp (GMPCPP) - blocked by a photo-labile group |
| 6-Methylthio-GTP | moderately promotive |
| 8Br-GTP | moderately promotive |
| dGTP | moderately promotive |
| mant-GTP | slightly inhibitory |
| XTP | slightly inhibitory |
| 6-Thio-GTP | highly inhibitory |
Läppchen et al. (2005) GTP Analogue Inhibits Polymerization and GTPase Activity of the Bacterial Protein FtsZ without Affecting Its Eukaryotic Homologue Tubulin. Biochemistry 44 (21):7879.
Muraoka et al. (1999) Effects of Purinenucleotide Analogues on Microtubule Assembly. Cell Structure and Function 24:305.
Hyman et al. (1992) Role of GTP Hydrolysis in Microtubule Dynamics: Information from a Slowly Hydrolyzable Analogue, GMPCPP. Mol. Biol. of the Cell 3:1155, and references therein.
