Microtubules and nucleoside diphosphate kinase. Nucleoside diphosphate kinase binds to co-purifying contaminants rather than to microtubule proteins

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Abstract
Nucleoside diphosphate (NDP) kinase has been postulated to generate GTP from the GDP bound to tubulin. The purified chick brain enzyme was studied with respect to its kinetic parameters, and the protein-protein interactions between the NDP kinase and tubulin were examined. No specific interaction is observed between the enzyme and (i) assembled microtubules, (ii) tubulin dimers, or (iii) tubulin-microtubule-associated protein (MAP) oligomers under a variety of nucleotide conditions. The apparent association is demonstrated to result from NDP kinase binding to a co-purifying contaminant. The absence of detectable NDP kinase-tubulin interactions indicates that NDP kinase does not directly charge up tubulin-GDP.