Bundled actin structures play a key role in maintaining cellular shape, in aiding force transmission to and from extracellular substrates, and in affecting cellular motility. Recent studies have also brought to light new details on stress generation, force transmission and contractility of actin bundles. In this work, we are primarily interested in the question of what determines the stability of actin bundles and what network geometries do unstable bundles eventually transition to. To address this problem, we used the MEDYAN mechano-chemical force field, modeling several micron-long actin bundles in 3D, while accounting for a comprehensive set of chemical, mechanical and transport processes. We developed a hierarchical clustering algorithm for classification of the different long time scale morphologies in our study. Our main finding is that initially unipolar bundles are significantly more stable compared with an apolar initial configuration. Filaments within the latter bundles slide easily with respect to each other due to myosin activity, producing a loose network that can be subsequently severely distorted. At high myosin concentrations, a morphological transition to aster-like geometries was observed. We also investigated how actin treadmilling rates influence bundle dynamics, and found that enhanced treadmilling leads to network fragmentation and disintegration, while this process is opposed by myosin and crosslinking activities. Interestingly, treadmilling bundles with an initial apolar geometry eventually evolve to a whole gamut of network morphologies based on relative positions of filament ends, such as sarcomere-like organization. We found that apolar bundles show a remarkable sensitivity to environmental conditions, which may be important in enabling rapid cytoskeletal structural reorganization and adaptation in response to intracellular and extracellular cues.
We provide evidence supporting that protein-protein and protein-ligand docking poses are functions of protein shape and intrinsic dynamics. Over sets of 68 protein-protein complexes and 240 nonhomologous enzymes, we recognize common predispositions for binding sites to have minimal vibrations and angular momenta, while two interacting proteins orient so as to maximize the angle between their rotation/bending axes (>65°). The findings are then used to define quantitative criteria to filter out docking decoys less likely to be the near-native poses; hence, the chances to find near-native hits can be doubled. With the novel approach to partition a protein into "domains" of robust but disparate intrinsic dynamics, 90% of catalytic residues in enzymes can be found within the first 50% of the residues closest to the interface of these dynamics domains. The results suggest an anisotropic rather than isotropic distribution of catalytic residues near the mass centers of enzymes.
Structure-encoded conformational dynamics are crucial for biomolecular functions. However, there is insufficient evidence to support the notion that dynamics play a role in guiding protein-nucleic acid interactions. Here, we show that protein-DNA docking orientation is a function of protein intrinsic dynamics, but the binding site itself does not display unique patterns in the examined spectrum of motions. This revelation is made possible by a novel technique that locates "dynamics interfaces" in proteins across which protein parts are anticorrelated in their slowest dynamics. A striking statistic is that such interfaces intersect the DNA in 97% of the 104 examined cases. These findings were then used to screen decoys generated by rigid-body docking of DNA molecules onto DNA-binding proteins. Using our method, the chance to discern near-native poses from non-native decoys increased by 2.5- and 1.6-fold, as compared to a random guess and methods based on surface complementarity, respectively. Hence, dynamically allowed protein-DNA docking orientations can work as new filters to cull and rerank docking poses and therefore enhance the predictability of DNA-binding sites that themselves do not have distinct dynamics features. Computer software implementing the method can be accessed via http://dyn.life.nthu.edu.tw/IDD/DNA.htm .
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, 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 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 parallel actin filaments that grow from their tips. Growing bundles zipper together upon contact with one another, an effect which is mediated by the surface tension of the liquid-like VASP droplets that encapsulate them. 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.
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