Lin Congping, Ashwin Peter, Steinberg Gero
Mathematics Research Institute, University of Exeter, Exeter, United Kingdom, EX4 4QF.
Phys Rev E Stat Nonlin Soft Matter Phys. 2013 May;87(5):052709. doi: 10.1103/PhysRevE.87.052709. Epub 2013 May 16.
Long-distance bidirectional transport of organelles depends on the coordinated motion of various motor proteins on the cytoskeleton. Recent quantitative live cell imaging in the elongated hyphal cells of Ustilago maydis has demonstrated that long-range motility of motors and their endosomal cargo occurs on unipolar microtubules (MTs) near the extremities of the cell. These MTs are bundled into antipolar bundles within the central part of the cell. Dynein and kinesin-3 motors coordinate their activity to move early endosomes (EEs) in a bidirectional fashion where dynein drives motility towards MT minus ends and kinesin towards MT plus ends. Although this means that one can easily assign the drivers of bidirectional motion in the unipolar section, the bipolar orientations in the bundle mean that it is possible for either motor to drive motion in either direction. In this paper we use a multilane asymmetric simple exclusion process modeling approach to simulate and investigate phases of bidirectional motility in a minimal model of an antipolar MT bundle. In our model, EE cargos (particles) change direction on each MT with a turning rate Ω and there is switching between MTs in the bundle at the minus ends. At these ends, particles can hop between MTs with rate q(1) on passing from a unipolar to a bipolar section (the obstacle-induced switching rate) or q(2) on passing in the other direction (the end-induced switching rate). By a combination of numerical simulations and mean-field approximations, we investigate the distribution of particles along the MTs for different values of these parameters and of Θ, the overall density of particles within this closed system. We find that even if Θ is low, the system can exhibit a variety of phases with shocks in the density profiles near plus and minus ends caused by queuing of particles. We discuss how the parameters influence the type of particle that dominates active transport in the bundle.
细胞器的长距离双向运输依赖于细胞骨架上各种运动蛋白的协同运动。最近在玉米黑粉菌细长的菌丝细胞中进行的定量活细胞成像表明，运动蛋白及其内体货物的长距离运动发生在细胞末端附近的单极微管（MTs）上。这些微管在细胞中央部分聚集成反极束。动力蛋白和驱动蛋白-3运动蛋白协调它们的活动，以双向方式移动早期内体（EEs），其中动力蛋白驱动向微管负端的运动，驱动蛋白驱动向微管正端的运动。虽然这意味着在单极部分可以很容易地确定双向运动的驱动者，但束中的双极方向意味着任何一种运动蛋白都有可能驱动向任何一个方向的运动。在本文中，我们使用多车道非对称简单排除过程建模方法，在反极微管束的最小模型中模拟和研究双向运动的阶段。在我们的模型中，EE货物（颗粒）在每个微管上以转向速率Ω改变方向，并且在束的负端微管之间存在切换。在这些末端，颗粒从单极部分进入双极部分时可以以速率q(1)在微管之间跳跃（障碍诱导切换速率），或者在相反方向通过时以速率q(2)跳跃（末端诱导切换速率）。通过数值模拟和平均场近似相结合，我们研究了这些参数以及封闭系统内颗粒的整体密度Θ的不同值下颗粒沿微管的分布。我们发现，即使Θ很低，系统也可以表现出各种相，在正端和负端附近的密度分布中由于颗粒排队而出现冲击。我们讨论了这些参数如何影响束中主导主动运输的颗粒类型。
