Abstract
The heterodimer tubulin is an essential protein for cell division, transport, cell motility and cell structure. The dimers can polymerize in the presence of Mg2+ and GTP to microtubules. The in vivo polymerization rate is regulated by microtubule associated proteins (MAPs).
Here we compare the kinetics of microtubule formation catalysed by glycerol and by the MAP Tau, respectively. While glycerol is responsible to support the polymerization of tubulin to single microtubules, Tau is known to form bundled microtubules. In this study we can show with different methods (turbidity measurements, small angle X-ray solution scattering and electron microscopy), that Tau is not only responsible for the tubulin polymerization to bundled microtubules, but also for the formation of tubulin rings. These tubulin rings have a 12-14fold symmetry and appear in a time scale of one minute. The rate constant of the ring formation is about 10 times faster than the rate constant for the polymerization (1×10−2 s−1 vs. 1×10−3 s−1 for 30 μM Tau and 18 μM tubulin).