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Tubulin Assembling Nucleotides

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.

Properties of tubulin-assembling nucleotides

Nucleotide analogProperties
ITPhighly promotive
GTPstandard 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-GTPmoderately promotive
8Br-GTPmoderately promotive
dGTPmoderately promotive
mant-GTPslightly inhibitory
XTPslightly inhibitory
6-Thio-GTPhighly inhibitory

Selected References

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.