Living Matter Department, AMOLF, Science Park 104, 1098 XG Amsterdam, The Netherlands.
Soft Matter. 2019 Apr 3;15(14):3036-3042. doi: 10.1039/c8sm02429d.
Transiently crosslinked actin filament networks allow cells to combine elastic rigidity with the ability to deform viscoelastically. Theoretical models of semiflexible polymer networks predict that the crosslinker unbinding rate governs the timescale beyond which viscoelastic flow occurs. However a direct comparison between network and crosslinker dynamics is lacking. Here we measure the network's stress relaxation timescale using rheology and the lifetime of bound crosslinkers using fluorescence recovery after photobleaching (FRAP). Intriguingly, we observe that the crosslinker unbinding rate measured by FRAP is more than an order of magnitude slower than the rate measured by rheology. We rationalize this difference with a three-state model where crosslinkers are bound to either 0, 1 or 2 filaments, which allows us to extract crosslinker transition rates that are otherwise difficult to access. We find that the unbinding rate of singly bound crosslinkers is nearly two orders of magnitude slower than for doubly bound ones. We attribute the increased unbinding rate of doubly bound crosslinkers to the high stiffness of biopolymers, which frustrates crosslinker binding.
瞬态交联肌动蛋白纤维网络使细胞能够将弹性刚性与粘弹性变形的能力结合起来。半刚性聚合物网络的理论模型预测，交联剂解联速率决定了发生粘弹性流动的时间尺度。然而，网络和交联剂动力学之间的直接比较尚缺乏。在这里，我们使用流变学测量网络的应力松弛时间尺度，使用光漂白后荧光恢复（FRAP）测量结合交联剂的寿命。有趣的是，我们观察到 FRAP 测量的交联剂解联速率比流变学测量的速率慢一个数量级以上。我们用一个三态模型来解释这种差异，其中交联剂要么结合到 0、1 或 2 根纤维上，这使得我们能够提取否则难以获得的交联剂转换速率。我们发现，单结合交联剂的解联速率比双结合交联剂的解联速率慢近两个数量级。我们将双结合交联剂的解联速率增加归因于生物聚合物的高刚性，这阻碍了交联剂的结合。
