Tubulin-associated nucleoside diphosphokinase.

Microtubule protein, prepared by cycles of polymerisation and dissociation, contained a nucleoside diphosphokinase (NDP kinase) activity (EC 2.7.4.6). This activity was not intrinsic to the tubulin dimer or the so-called microtubule-associated proteins. The NDP kinase had the following properties. (1) The enzyme existed in a low-molecular-weight form and in association with the complex of microtubule-associated proteins and tubulin (i.e. multimeric tubulin). (2) The low-molecular-weight species was also formed by dissociation of multimeric tubulin by salt or by removal of microtubule-associated proteins on phosphocellulose. (3) GDP bound to the exchangeable site of multimeric tubulin and also GDP derived from the E site of the tubulin dimer was a substrate for the NDP kinase. (4) The NDP kinase showed a 7-fold increase in activity during ATP-dependent microtubule assembly. On the basis of these properties, it is proposed that microtubule protein contains an NDP kinase specifically associated with tubulin and its functions.