Department of Chemistry, University of North Carolina, Chapel Hill, North Carolina, USA.
Biophys J. 2010 Apr 21;98(8):1439-48. doi: 10.1016/j.bpj.2009.12.4325.
Recent modeling of filopodia--the actin-based cell organelles employed for sensing and motility--reveals that one of the key limiting factors of filopodial length is diffusional transport of G-actin monomers to the polymerizing barbed ends. We have explored the possibility of active transport of G-actin by myosin motors, which would be an expected biological response to overcome the limitation of a diffusion-based process. We found that in a straightforward implementation of active transport the increase in length was unimpressive, < or = 30%, due to sequestering of G-actin by freely diffusing motors. However, artificially removing motor sequestration reactions led to approximately threefold increases in filopodial length, with the transport being mainly limited by the motors failing to detach from the filaments near the tip, clogging the cooperative conveyer belt dynamics. Making motors sterically transparent led to a qualitative change of the dynamics to a different regime of steady growth without a stationary length. Having identified sequestration and clogging as ubiquitous constraints to motor-driven transport, we devised and tested a speculative means to sidestep these limitations in filopodia by employing cross-linking and putative scaffolding roles of Ena/VASP proteins. We conclude that a naïve design of molecular-motor-based active transport would almost always be inefficient--an intricately organized kinetic scheme, with finely tuned rate constants, is required to achieve high-flux transport.
最近对丝状伪足（用于感知和运动的肌动蛋白细胞器官）的建模表明，丝状伪足长度的一个关键限制因素是 G 肌动蛋白单体向聚合的快末端扩散运输。我们已经探索了肌球蛋白马达主动运输 G 肌动蛋白的可能性，这将是克服基于扩散过程的限制的预期生物学反应。我们发现，在主动运输的直接实现中，由于自由扩散的马达隔离 G 肌动蛋白，长度的增加不明显，<或=30%。然而，人为地去除马达隔离反应导致丝状伪足长度增加约三倍，运输主要受到靠近尖端的纤维上的马达无法脱离的限制，堵塞了协作输送带动力学。使马达具有空间透明性导致动力学发生定性变化，进入稳定生长而没有静止长度的不同状态。由于确定了隔离和堵塞是马达驱动运输的普遍限制，我们设计并测试了一种推测性的方法，通过交联和 Ena/VASP 蛋白的假定支架作用来规避丝状伪足中的这些限制。我们得出结论，基于分子马达的主动运输的天真设计几乎总是效率低下的——需要复杂的组织动力学方案，精细调整速率常数，以实现高通量运输。
