DIRECT IMAGING OF REPTATION FOR SEMIFLEXIBLE ACTIN-FILAMENTS

ACCORDING to the reptation model of polymer diffusion(1), a polymer ch ain exhibits snake-like motion through the entangled mesh of surroundi ng molecules, in which the undulations of the chain are restricted to a tubelike region(2). The reptation model can account for many of the dynamic properties of entangled polymer solutions and melts, and has r eceived support from observations of block copolymer diffusion across an interface(3); but reptative motion has not previously been imaged d irectly(4,5). Here we report such a direct observation of reptation, o btained by video microscopy of fluorescently labelled single, semiflex ible filaments of actin in a solution of unlabelled actin filaments. F rom the restricted thermal undulations of these filaments we can measu re the diameter of the confining tube, and we also observe the charact eristic thermally excited sliding of the filament out of the end of th e tube. We find that the chain self-diffusion coefficient decreases ap proximately linearly as the filament length increases, in agreement wi th the reptation model.