Abstract
Filamentous actin is one of the most important cytoskeletal elements. Not only is it responsible for the elastic properties of many cell types, but it also plays a vital role in cellular adhesion and motility. Understanding the bundling kinetics of actin filaments is important in the formation of various cytoskeletal structures, such as filopodia and stress fibers. Utilizing a unique pillar-structured microfluidic device, we investigated the time dependence of bundling kinetics of pillar supported free-standing actin filaments. Microparticles attached to the filaments allowed the measurement of thermal motion, and we found that bundling takes place at lower concentrations than previously found in 3-dimensional actin gels, i.e. actin filaments formed bundles in the presence of 5–12 mM of magnesium chloride in a time-dependent manner. The filaments also displayed long term stability for up to hours after removing the magnesium ions from the buffer, which suggests that there is an extensive hysteresis between cation induced crosslinking and decrosslinking.
Highlights
Actin is one of the most abundant cytoskeletal proteins with crucial roles in maintaining the cellular shape, elasticity, adhesion and motility [1,2,3]
Due to the nature of this construction method, the actual orientation could not be controlled, the parallel geometry contains a random set of actin filaments oriented in both parallel and ant-iparallel directions
In order to resolve the apparently contradicting results in the literature and within our own experiments, we hypothesized that the electrostatic interactions lead to the binding of magnesium to actin, whereby the ions may become entrapped between the filaments, resulting in an irreversible process
Summary
Actin is one of the most abundant cytoskeletal proteins with crucial roles in maintaining the cellular shape, elasticity, adhesion and motility [1,2,3]. In its natural monomer form it is a globular protein (G-actin), but in cells and under proper buffer conditions these globular units dynamically assemble into filaments (F-actin). Numerous previous studies have analyzed the morphology and rheological properties of actin gels. Actin filaments can reach a length of *20–30 μm. These studies show that the actin filaments possess a negative net charge with a linear charge density of about 4 e−/nm, a helical structure with a twisting increment of *36 nm/turn and a diameter of *7–9 nm [2, 4,5,6]. Actin filaments are semiflexible, with a persistence length in the order of 8–17 μm[7, 8], which is about 1/4–1/2 of their contour length, depending on the presence and type of the stabilization agent
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