PHASE-DIAGRAM OF MICROTUBULES

We map the phase diagram of microtubules as a function of temperature and tubulin concentration. We observe spontaneous and site-nucleated microtubule assembly. At temperatures and concentrations below the onset of spontaneous nucleation, we measure the steady-state proportion of occupied nucleation sites and the distribution of lengths of site-nucleated microtubules. Our observations reveal a transition in the length dynamics of microtubules from bounded to unbounded growth. This transition is also evident in the length dynamics of individual site-nucleated microtubules. The transition to spontaneous microtubule assembly completes the phase diagram. We measure the temperature and concentration dependence of the latent time for spontaneous nucleation of microtubules in the bulk.