ATP-induced aggregates of tubulin rings

Aggregates of double-walled rings (46 nm diameter) were formed upon warming (37 degrees C) ion exchange purified bovine brain tubulin (1.5 to 2.0 mg/ml) in the presence of 1.0 mM ATP and 5.0 mM Mg2+. The formation of aggregated rings was blocked by GDP (1.0 mM), but when GTP (1.0 mM) was added subsequently, the block was overcome. Warming in the presence of ATP and GTP simultaneously produced a mixture of microtubules and aggregated rings. The tubulin preparations used were demonstrated to be devoid of transphosphorylase, and it is therefore clear that the action of ATP in the induction of aggregated rings was not mediated by transphosphorylation. These results are consistent with an interaction of nucleotide with tubulin at a site distinct from the previously described N- and E-sites.