Actin Bundles

Filamentous actin bundles are key components for the mechanical support of eukaryotic cells. Bundles integrated into the cytoskeleton are designed principally to reinforce the cell against mechanical deformation and to allow for force generation: Dynamic filopodia are needed to exert protrusion forces at leading edge of a motile cell, microvilli are passive structural elements that increase the surface area of the cells, stress fibers are essential for cell adhesion to the substratum and for changes in cell morphology. In vivo, F-actin is bundled by a myriad of actin-binding proteins (ABPs) that enable the cell to tailor bundle dimensions and mechanical properties to suit the variety of biological functions including cell motility, mechanosensation, and phagocytosis. In each case, the cell uses a specific set of ABPs to adapt the bundles to the particular tasks:α-actinin (A) predominates in cytoskeletal bundles and muscle, filamin (B) is ubiquitous with signalling function, fascin (C) is prevalent in filopodia, and plastin and espin (D) are predominant in microvilli and stereocilia. Using several in vitro model systems, we investigate the structure and dynamics of different bundle types.

How are bundles organized?

Each cytoskeletal process employs its own set of ABPs which often results in a well defined length and thickness of the bundles. We have shown that - for the ABPs fascin and espin - a maximal bundle size exists and were able to quantify the numbers of filaments within a bundle using an emulsion droplet system. To investigate the microscopic bundle geometry more closely, we performed Small-Angle-X-Ray (SAXS) experiments. This revealed that the mismatch between the helical structure of a single filament and the hexagonal packing symmetry within the bundle is essential for the control of bundle thickness. The energetic tradeoff between filament twisting and ABP binding is a fundamental mechanism by which cells can precisely adjust bundle size and strength.

How is the stiffness of a single bundle manipulated by different ABPs?

The disparate mechanical requirements of the cellular processes, together with the broad evolutionary conservation of their predominant ABPs across vertebrate and invertebrate eukaryotes, suggests that the ability of an ABP to differentially mediate the bundle stiffness is a key component of its biological function. Two limiting types of bundle bending have been reported: decoupled bending (A), in which the filaments bend independently because intervening ABPs do not resist shear during bundle bending, and fully coupled bending (B), in which filaments are rigidly "glued" together forcing filaments away from the bundle neutral surface to additionally stretch or compress. Using an emulsion droplet system (C) we see that the ABP type clearly affects the degree of shear coupling between the filaments in the bundle depending on the finite shear stiffness of the ABP.


What is the microscopic origin of the elasticity of bundled networks?

The local elastic properties in a cell are tightly regulated by the activation of ABPs which crosslink or bundle the actin filaments into complex networks. Given the importance of the actin cytoskeleton for force generation and transduction there is much interest in understanding the mechanical properties of different network structures and the physical origin of the transition between them. Crosslinked semiflexible polymer networks are in general dominated by stretching single filaments (A). In a purely bundled system, the thermal excess length is highly diminished as this is inversely proportional to the persistence length of the polymer, i.e. the bundle (B). We have shown, that the concept of the "floppy modes" is suited to describe the polymer elasticity in networks which are highly bundled by fascin.