DIRECT MEASUREMENT OF STIFFNESS OF SINGLE ACTIN-FILAMENTS WITH AND WITHOUT TROPOMYOSIN BY IN-VITRO NANOMANIPULATION

In order to explain the molecular mechanism of muscle contraction, it is crucial to know the distribution of the sarcomere compliance of act ive muscle. Here, we directly measure the stiffness of single actin fi laments with and without tropomyosin, using a recently developed techn ique for nanomanipulation of single actin filaments with microneedles. The results show that the stiffness for 1-mu m-long actin filaments w ith and without tropomyosin is 65.3 +/- 6.3 and 43.7 +/- 4.6 pN/nm, re spectively, When the distribution of crossbridge forces along the acti n filament is taken into account, the elongation of a 1-mu m-long thin filament during development of isometric contraction is calculated to be approximate to 0.23%. The time constant of force in response to a sudden length change is <0.2 ms, indicating that the viscoelasticity i s negligible in the millisecond time range. These results suggest that approximate to 50% of the sarcomere compliance of active muscle is du e to extensibility of the thin filaments.