Fig. 1.

Actin filament growth from isolated Limulus acrosomal bundles. (a) Limulus acrosomal bundle fragments (≈5 μm long) were incubated with rhodamine-actin. Small tufts of fluorescent actin filaments are seen growing from the barbed end in this sequence of phase-contrast, rhodamine, and overlay images. Notice that some acrosomes form clusters. (b) Electron micrographs of negatively stained filaments grown from acrosomal bundles with 4 μM monomeric actin and 20 μM profilin. (Upper) Ten seconds of growth. (Lower) Thirty seconds of growth. (c) Apparent actin critical concentration measured for different concentrations of profilin (actin/profilin ratio 1:5 for all experiments). Duplicate points show results of independent experiments. (d) The number of filaments per acrosome, counted during the first 40 s of growth. The difference in the number of filaments per acrosome at different time points was not significant. (e) Diffusivity in length of the actin filaments in the three assay conditions over the first 40 s of growth. The complete set of diffusivity data are presented in SI Fig. 8.