Department of Chemistry, Yale University, New Haven, Connecticut; Department of Molecular Biophysics and Biochemistry, Yale University, New Haven, Connecticut.
Department of Molecular Biophysics and Biochemistry, Yale University, New Haven, Connecticut; Department of Physics, Yale University, New Haven, Connecticut.
Biophys J. 2019 Dec 3;117(11):2066-2078. doi: 10.1016/j.bpj.2019.10.027. Epub 2019 Oct 25.
Microtubules are dynamic cytoskeletal polymers whose growth and shrinkage are highly regulated as eukaryotic cells change shape, move, and divide. One family of microtubule regulators includes the ATP-hydrolyzing enzymes spastin, katanin, and fidgetin, which sever microtubule polymers into shorter fragments. Paradoxically, severases can increase microtubule number and mass in cells. Recent work with purified spastin and katanin accounts for this phenotype by showing that, in addition to severing, these enzymes modulate microtubule dynamics by accelerating the conversion of microtubules from their shrinking to their growing states and thereby promoting their regrowth. This leads to the observed exponential increase in microtubule mass. Spastin also influences the steady-state distribution of microtubule lengths, changing it from an exponential, as predicted by models of microtubule dynamic instability, to a peaked distribution. This effect of severing and regrowth by spastin on the microtubule length distribution has not been explained theoretically. To solve this problem, we formulated and solved a master equation for the time evolution of microtubule lengths in the presence of severing and microtubule dynamic instability. We then obtained numerical solutions to the steady-state length distribution and showed that the rate of severing and the speed of microtubule growth are the dominant parameters determining the steady-state length distribution. Furthermore, we found that the amplification rate is predicted to increase with severing, which is, to our knowledge, a new result. Our results establish a theoretical basis for how severing and dynamics together can serve to nucleate new microtubules, constituting a versatile mechanism to regulate microtubule length and mass.
微管是动态的细胞骨架聚合物，在真核细胞改变形状、运动和分裂时，其生长和收缩受到高度调控。微管调节因子家族包括 ATP 水解酶 spastin、katanin 和 fidgetin，它们将微管聚合物切成更短的片段。矛盾的是，切割酶可以增加细胞中的微管数量和质量。最近使用纯化的 spastin 和 katanin 的工作通过表明，除了切割之外，这些酶还通过加速微管从收缩状态到生长状态的转化来调节微管动力学，从而促进它们的重新生长。这导致观察到的微管质量呈指数增长。spastin 还影响微管长度的稳态分布，将其从微管动态不稳定性模型预测的指数分布改变为峰分布。spastin 对微管长度分布的切割和重新生长的这种影响在理论上尚未得到解释。为了解决这个问题，我们制定并解决了一个存在切割和微管动态不稳定性时微管长度时间演化的主方程。然后，我们得到了稳态长度分布的数值解，并表明切割速度和微管生长速度是决定稳态长度分布的主要参数。此外，我们发现预测的放大率随切割而增加，这在我们的知识范围内是一个新的结果。我们的结果为切割和动力学如何共同作为新微管的成核点提供了理论基础，构成了一种调节微管长度和质量的通用机制。
