Measurement of binding force of the catch complex. One end of an
F-actin filament (tandem arrowhead) was brought into contact with a
thick filament (arrowhead) to form the catch complex
(A), while the other end was captured with a needle
coated with N-ethylmaleimide-treated myosin (indicated
by “N”; stiffness of this needle = 18 pN/μm)
(B). Then, the microscope stage was moved at a constant
velocity of ≈1 μm/s to apply force on the catch complex by the
deflection of the needle (C and D). When
the deflection of the needle reached a critical extent (21 μm;
E), the F-actin filament abruptly detached from the
needle. The needle then moved back to the baseline under its own
elasticity (F). Even at the break of the bond between
the F-actin and the needle, the other end of the F-actin filament
(tandem arrowhead) still remained attached to the thick filament
(arrowhead) on the glass surface (G). Therefore, the
maximal deflection of the needle indicated the lower limit of the
binding force sustained by the overlap of the F-actin and thick
filaments (1.7 μm). The lower limit of the binding force per overlap
was calculated to be 220 pN/μm. (Bar = 20 μm.)