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Tubulin-nucleotide interactions during the polymerization and depolymerization of microtubules
Biochemistry
|September 21, 1976
Summary
Guanosine triphosphate (GTP) drives tubulin polymerization, but its hydrolysis is crucial for depolymerization, indicating an irreversible process. Nucleotide exchange at the E site is restricted during microtubule assembly and disassembly.
Area of Science:
- Biochemistry
- Cell Biology
- Molecular Dynamics
Background:
- Tubulin polymerization into microtubules is essential for cellular structure and function.
- Nucleotide interactions, particularly with guanosine triphosphate (GTP), regulate tubulin dynamics.
Purpose of the Study:
- To elucidate the role of nucleotide binding and hydrolysis in tubulin polymerization.
- To investigate the thermodynamic reversibility of microtubule assembly and disassembly.
Main Methods:
- Studied tubulin polymerization using GTP, adenosine triphosphate (ATP), and non-hydrolyzable GTP analogs (GMPPNP).
- Investigated nucleotide exchange dynamics on tubulin using radiolabeled GTP ([3H]GTP).
- Examined the effects of calcium ions (Ca) on microtubule assembly and disassembly.
Main Results:
- GTP binding promotes tubulin polymerization, with GTP hydrolysis occurring during or after assembly.
- ATP can transphosphorylate GDP to GTP, initiating polymerization after a lag period.
- GTP hydrolysis appears to promote depolymerization, and GMPPNP-assembled microtubules are more stable.
- The exchangeable nucleotide site (E site) on tubulin is blocked in microtubules and ring aggregates but becomes available during temperature-induced assembly/disassembly.
Conclusions:
- Tubulin polymerization is not a thermodynamically reversible process, involving irreversible steps.
- GTP hydrolysis, not polymerization, drives microtubule depolymerization.
- The blocked E site in tubulin aggregates explains nucleotide hydrolysis observations in other studies.