Biomechanics of an isolated single stress fiber

Knowledge of mechanical properties of stress fiber (SF), a bundle of actin filaments, is crucial for understanding its role in mechanotransduction in adherent cells. Here, we characterized tensile properties of single SFs by in vitro manipulation. SFs were isolated from cultured vascular smooth muscle cells with a combination of low ionic-strength extraction and detergent extraction and were stretched until breaking. The breaking force of the SFs for stretching was, on average, 377 nN, which was greater than actin filaments, 600 pN. The Young’s modulus was estimated as 1.45 MPa, which was three orders of magnitude lower than actin filaments. Strain-induced hardening, a common mechanical behavior of living adherent cells, was observed in a physiological strain range of the force-strain curves. Estimated force level of physiological tension in single SFs was the same order of magnitude with that of the substrate traction force of adherent cells required for maintenance of cell integrity. These results suggest that SFs are a principal subcellular component in bearing intracellular stresses.