Nucleotide specificity in microtubule assembly in vitro.

Area of Science:

• Biochemistry
• Cell Biology
• Molecular Biology

Background:

• Tubulin is the primary protein subunit of microtubules, essential for cell structure and division.
• The exchangeable GTP binding site (E site) on tubulin plays a critical role in microtubule dynamics.
• GDP binding at the E site typically inhibits microtubule polymerization.

Purpose of the Study:

• To investigate the role of GDP at the E site of tubulin in microtubule assembly.
• To determine the nucleotide specificity of the E site after GDP removal.
• To explore conditions that can induce microtubule polymerization.

Main Methods:

• A procedure was developed to remove GDP from the E site of tubulin.
• Microtubule protein with reduced GDP at the E site was polymerized.
• Polymerization was induced using nonhydrolyzable ATP analogs (AMP-PNP, AMP-PCP) and low concentrations of GTP or dGTP.

Main Results:

• Tubulin lacking most GDP at the E site polymerizes in response to nonhydrolyzable ATP analogs.
• Substoichiometric levels of GTP or dGTP also induce polymerization of GDP-depleted tubulin.
• The E site exhibits broad specificity for nucleoside triphosphates once GDP is removed.

Conclusions:

• Microtubule assembly can be initiated by the binding of substoichiometric amounts of nucleoside triphosphates to the E site.
• The removal of GDP from tubulin's E site reveals a broader nucleotide binding and polymerization-inducing capacity.
• This finding has implications for understanding microtubule regulation and potential therapeutic targets.