Graham Kristin, Chandrasekaran Aravind, Wang Liping, Ladak Aly, Lafer Eileen M, Rangamani Padmini, Stachowiak Jeanne C
University of Texas at Austin, Department of Biomedical Engineering.
University of California San Diego, Department of Mechanical and Aerospace Engineering.
Nat Phys. 2023 Apr;19(4):574-585. doi: 10.1038/s41567-022-01924-1. Epub 2023 Jan 30.
The organization of actin filaments into bundles is required for cellular processes such as motility, morphogenesis, and cell division. Filament bundling is controlled by a network of actin-binding proteins. Recently, several proteins that comprise this network have been found to undergo liquid-liquid phase separation. How might liquid-like condensates contribute to filament bundling? Here, we show that the processive actin polymerase and bundling protein, VASP, forms liquid-like droplets under physiological conditions. As actin polymerizes within VASP droplets, elongating filaments partition to the edges of the droplet to minimize filament curvature, forming an actin-rich ring within the droplet. The rigidity of this ring is balanced by the droplet's surface tension, as predicted by a continuum-scale computational model. However, as actin polymerizes and the ring grows thicker, its rigidity increases and eventually overcomes the surface tension of the droplet, deforming into a linear bundle. The resulting bundles contain long, parallel actin filaments that grow from their tips. Significantly, the fluid nature of the droplets is critical for bundling, as more solid droplets resist deformation, preventing filaments from rearranging to form bundles. Once the parallel arrangement of filaments is created within a VASP droplet, it propagates through the addition of new actin monomers to achieve a length that is many times greater than the initial droplet. This droplet-based mechanism of bundling may be relevant to the assembly of cellular architectures rich in parallel actin filaments, such as filopodia, stress fibers, and focal adhesions.
肌动蛋白丝束的形成对于诸如运动、形态发生和细胞分裂等细胞过程是必需的。丝束的形成由肌动蛋白结合蛋白网络控制。最近，人们发现构成该网络的几种蛋白质会发生液-液相分离。类液凝聚物如何促进丝束的形成呢？在这里，我们表明，持续性肌动蛋白聚合酶和束集蛋白VASP在生理条件下会形成类液滴。当肌动蛋白在VASP液滴内聚合时，伸长的丝会分配到液滴边缘以使丝的曲率最小化，从而在液滴内形成富含肌动蛋白的环。正如一个连续尺度计算模型所预测的那样，这个环的刚性由液滴的表面张力平衡。然而，随着肌动蛋白聚合且环变厚，其刚性增加并最终克服液滴的表面张力，变形为线性束。所形成的束包含从其末端生长的长而平行的肌动蛋白丝。重要的是，液滴的流体性质对于束集至关重要，因为更固态的液滴会抵抗变形，阻止丝重新排列形成束。一旦在VASP液滴内形成了丝的平行排列，它会通过添加新的肌动蛋白单体进行扩展，以达到比初始液滴大许多倍的长度。这种基于液滴的束集机制可能与富含平行肌动蛋白丝的细胞结构的组装有关，如丝状伪足、应力纤维和粘着斑。
