Tubulin‐Associated Nucleoside Diphosphokinase

Abstract

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.