Light microscopic analysis of ligand-induced actin filament suprastructures

In this study, we describe a simple light microscopic assay which allows to rapidly determine the ligand-induced organization of actin filaments into specific suprastructures, such as web-like arrangements or bundles. The validity of this assay is demonstrated by accompanying low shear (falling ball) viscometry. While the visually identified webs demonstrated viscosity values significantly higher than the F-actin control, the bundles were characterized by viscosities distinctly lower than that of the control. In addition, we show that at least in some cases, the type of actin suprastructure formed depends on the molar ratio between the ligand and actin filaments. The assay should be useful to assess the conditions under which a particular ligand leads to a specific actin filament organization, to determine quickly the biological activity of recombinant proteins or isolated actin-binding domains, and to define new F-actin ligands.