Abstract
A quantitative analysis of the interplay between guanosine 5'-triphosphate (GTP) and guanosine 5'-diphosphate (GDP) in microtubule assembly and accompanying GTP hydrolysis has been performed when tubulin was polymerized in the presence of microtubule-associated proteins (MAPs) which display an interfering GTPase activity. The use of adenylyl beta-imidodiphosphate, which specifically inhibits the MAPs GTPase activity, and of vinblastine (or podophyllotoxin), which specifically inhibits GTP hydrolysis due to tubulin, made possible a study of the extensive GTP hydrolysis associated to microtubule assembly. The results indicate that GDP binds to microtubule ends with an affinity comparable to GTP, thus strongly inhibiting both the elongation process and the steady-state GTP hydrolysis at microtubule ends. GDP shifts the equilibrium between tubulin and microtubules toward disassembly. The MAPs which are released from the microtubules during the GDP-driven depolymerization cluster on the remaining microtubules. The resulting increased stability of microtubules is quantitatively consistent with the decrease in the critical concentration of the polymerizing species GTP-tubulin.
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