Methods to Characterize Actin Filament Networks

This chapter covers methods used to characterize actin filament networks. The interest in these networks stems from studies of cell extracts and purified proteins that show that actin filaments can be crosslinked together to form a gel. Many early studies on intact cells suggested that such gels and the gel–sol transition are important for both cytoplasmic structure and motility. A number of different assays have been used to characterize the physical properties of these actin-filament liquids and gels and to assay the crosslinking and regulatory proteins. The critical gelling concentration can be estimated with very simple assays, while the evaluation of viscous and elastic moduli as a function of shear stress requires more sophisticated equipment. The assays fall into three categories: (a) test tube inversion, low speed sedimentation, and falling ball viscometry, which are simple enough to use routinely to monitor the purification of gelation factors and regulatory proteins; (b) the falling-ball assay and a gelmeter, which can be used for semiquantitative analysis of gelation; (c) several additional assays that require more complex equipment and are available for quantitative analysis of the physical properties of the gels.