Caplow M, Shanks J, Breidenbach S, Ruhlen R L
Department of Biochemistry, University of North Carolina, Chapel Hill 27599-7260.
J Biol Chem. 1988 Aug 5;263(22):10943-51.
Microtubules at steady state were found to undergo dramatic changes in length, with only very little change in number concentration and mean length. This result is accounted for by a mechanism in which microtubules are capped at ends by tubulin-GTP subunits; loss of the tubulin-GTP cap at one end results in disassembly of all the tubulin-GDP subunits, so that the medial edge of the distal tubulin-GTP cap is exposed; the exposed tubulin-GTP cap is sufficiently stable, so that microtubule regrowth from the cap rather than loss of the cap occurs. This mechanism predicts that a bell-shaped length distribution of sheared microtubules will be transiently bimodal, with peaks of short and moderate length microtubules, in rearranging to an exponential length distribution. We have observed the predicted transient bimodal length distribution experimentally and in a Monte Carlo simulation. Dynamic instability has recently been accounted for by assuming that microtubule ends are capped with only a single tubulin-GTP subunit at each end of the five helices that serve as elongation sites. Such a minimal tubulin-GTP cap is apparently ruled out by our observations, which require that the remnant tubulin-GTP cap generated from disassembly be able to serve as nucleating site; we do not expect that a stable nucleating site can be generated from five tubulin-GTP subunits, oriented as the five helices that serve as elongation sites.
在稳态下，微管的长度发生了显著变化，而数量浓度和平均长度变化很小。这一结果可由一种机制来解释，即微管两端由微管蛋白 - GTP亚基封端；一端微管蛋白 - GTP帽的丢失会导致所有微管蛋白 - GDP亚基的解聚，从而使远端微管蛋白 - GTP帽的内侧边缘暴露；暴露的微管蛋白 - GTP帽足够稳定，因此微管从帽处重新生长而非帽丢失。该机制预测，剪切后的微管呈钟形的长度分布在重排为指数长度分布时将暂时呈现双峰，即短长度和中等长度微管的峰值。我们已通过实验和蒙特卡罗模拟观察到了预测的暂态双峰长度分布。最近，动态不稳定性是通过假设微管末端在作为延伸位点的五个螺旋的每一端仅由单个微管蛋白 - GTP亚基封端来解释的。我们的观察结果显然排除了这种最小的微管蛋白 - GTP帽，因为我们的观察要求从解聚产生的残余微管蛋白 - GTP帽能够作为成核位点；我们预计由五个微管蛋白 - GTP亚基（如作为延伸位点的五个螺旋那样排列）无法产生稳定的成核位点。
