Figure 5.

Hypothesis for MT treadmilling. (A) MT dynamics pattern. The probability that a MT end is growing is plotted versus tubulin subunit concentration. When minus ends are capped, the steady-state subunit concentration is set by the critical concentration for the MT plus end, c₊. Below this value, both plus and minus ends of MTs shorten. At c₊, plus ends (•) show dynamic instability—they both grow and shorten—whereas any free minus ends (○) only shorten. If minus ends are uncapped (−centrosome), tubulin subunits released from shortening minus ends will increase the tubulin pool until plus ends persistently grow and a concentration is reached, c_TM, at which MTs display balanced treadmilling (TM). Above this value, growth could theoretically occur at minus ends (dashed line), but the range at which this occurs in vivo has not been determined. (B) Intrinsic vs. facilitated treadmilling. MT treadmilling is rate limited by dissociation of tubulin subunits from the minus end. Treadmilling in vitro is slow because this rate is low. In living cells, minus-end shortening is 2–3 orders of magnitude faster. Protein factors may bind to the minus end, facilitating loss of subunits. This would increase treadmilling speed.