Cytoskeletal Mechanics

Figure 1: Fluorescently labeled actin (100-fold excess of unlabeled actin) crosslinked by heavy mero myosin, scale bar 50µm; inset: schematic actin network.

Force-dependent properties of individual actin crosslinkers

Cells need strong structures providing mechanical stability to withstand mechanical stress. At the same time these structures should not be too rigid so that the cell can reorganize relatively quickly to change its shape e.g. for locomotion. The highly dynamic cytoskeleton is a powerful tool, enabling the cell to meet these conflicting demands. Although it is mainly composed of actin filaments, microtubules and intermediate filaments, the properties of the whole cytoskeletal network is not only defined by the properties of these filaments, but can be precisely modulated by various crosslinking and motor proteins binding to the filaments.

 

While the mechanical and rheological properties of crosslinked actin networks have already been extensively investigated in ensemble studies, detailed information on the characteristics of individual crosslinks is still rare.

 

Figure 2: Schematic and fluorescent image of the assay (beads and actin filaments fluorescently labeled), scale bar 5µm.

 

We have developed a four bead optical tweezers assay to probe the force-dependent properties of individual, freely suspended actin-crosslinker-actin bonds. This method has already shed light on how mechanical forces influence the dynamics and strength of actin crosslinks.

 

Figure 3: Optical setup for fluorescence microscopy allowing to visualize single actin filaments as shown in Figure 1 and 2.

 

Studying forces acting in muscle on the molecular level

In general the unfolding pathway of thermal or chemical denaturation of a protein can differ significantly from mechanical unfolding pathways. Since many proteins are exposed to notable forces in their in vivo environment investigation of their mechanical stability can yield new insights about their physiological function on the molecular level. The muscle Z-disc for instance is a cellular structure constantly subject to considerable mechanical stress. Untill now it has largely remained unknown how the proteins of the Z-disc are adapted to the forces acting in muscle. Thus, our current research investigates the mechanical stability of important molecular structures in the muscle Z-disc on the single molecule level using an AFM-setup.