Rapid treadmilling of brain microtubules free of microtubule-associated proteins in vitro and its suppression by tau

We have determined the treadmilling rate of brain microtubules (MTs) free o f MT-associated proteins (MAPs) at polymer mass steady state in vitro by us ing [H-3]GTP-exchange. We developed buffer conditions that suppressed dynam ic instability behavior by approximate to 10-fold to minimize the contribut ion of dynamic instability to total tubulin-GTP exchange. The MTs treadmill ed rapidly under the suppressed dynamic instability conditions, at a minimu m rate of 0.2 mu m/min. Thus, rapid treadmilling is an intrinsic property o f MAP-free MTs, Further, we show that tau. an axonal stabilizing MAP involv ed in Alzheimer's disease, strongly suppresses the treadmilling rate. These results indicate that tau's function in axons might involve suppression of axonal MT treadmilling. We describe mathematically how treadmilling and dy namic instability are mechanistically distinct MT behaviors. Finally, we pr esent a model that explains how small changes in the critical tubulin subun it concentration at MT minus ends, caused by intrinsic: differences in rate constants or regulatory proteins, could produce large changes in the tread milling rate.