Microtubule treadmills--possible molecular machinery.

Microtubule polymerization in vitro is the summation of different reactions occurring at each end of the polymer. In steady-state conditions in vitro, net tubulin addition on the microtubule occurs at one end of the polymer, and net tubulin loss occurs at the opposite end. Thus, a unidirectional flux of tubulin from one end of the microtubule to the other, or "treadmilling', can occur. The opposite end assembly--disassembly behaviour of microtubules, if it occurs within cells, could be fundamentally linked to the functions of microtubules, as, for example, in the translocation of chromosomes during mitosis.