DIRECT MEASUREMENT OF THE WAVE-VECTOR-DEPENDENT BENDING STIFFNESS OF FREELY FLICKERING ACTIN-FILAMENTS

Fourier analysis of the thermally excited undulations of fluorescence-labelled actin filaments allowed for the first time wave-vector-dependent measurements of the bending elastic modulus k(c). At wave vectors q > 2 . 10(6) m-1, scaling behaviour characteristic for bending undulations is clearly observed yielding a bending modulus of k(c) = (4.0 +/- 0.4).10(-27) Jm (corresponding to a persistence length L(p) = 0.5 mum). For decreasing wave vector q the filament stiffness increases continuously. The wave-vector-dependent filament stiffness explains the differences of the persistence length as determined by static end-to-end distance measurements (L(p) = (6 +/- 25) mum) and by dynamic techniques (e.g., rheology and quasi-elastic light scattering) yielding L(p) = 0.5 mum. The filaments exhibit occasional bends which are attributed to defects in the double-stranded threads. Binding of tropomyosin leads to a decrease of the measured bending elasticity to k(c) = (1.11 +/- 0.09) . 10(-27) Jm for wave vectors q > 2 . 10(6)m-1. The course of the wave vector dependence of the bending stiffness remains the same.