Annealing accounts for the length of actin filaments formed by spontaneouspolymerization

We measured the lengths of actin filaments formed by spontaneous polymeriza tion of highly purified actin monomers by fluorescence microscopy after lab eling with rhodamine-phalloidin. The length distributions are exponential w ith a mean of similar to 7 mu m (2600 subunits). This length is independent of the initial concentration of actin monomer, an observation inconsistent with a simple nucleation-elongation mechanism. However, with the addition of physically reasonable rates of filament annealing and fragmenting, a nuc leation-elongation mechanism can reproduce the observed average length of f ilaments in two types of experiments: 1) filaments formed from a wide range of highly purified actin monomer concentrations, and 2) filaments formed f rom 24 mu M actin over a range of CapZ concentrations.