Actin assembly and filament cross‐linking in the presence of TW 260/240, the tissue‐specific spectrin of the chicken intestinal brush border

TW 260/240 is a tissue‐specific spectrin found in the terminal web region of the chicken intestinal bruish border. We have examined the effects of TW 260/240 on assembly rates and critical concentrations (C_o's) for monomer addition at the barbed and pointed ends of the actin filament. For these studies, acrosomal processes (AP) from Limulus sperm were used as nuclei for actin assembly. Under conditions which favor the interaction of TW 260/240 for actin (20–75 mM KCl, 2 mM Mg⁺⁺) no effect on either elongation rates or C_o's at either end of the actin filament was observed in the presence of this spectrinlike protein. The Limulus AP nucleation assay also allowed visualization of the kinetics of filament binding and cross‐linking by TW 260/240. Ultrastructural analysis of TW 260/240 binding to actin filaments at their growing ends indicates that TW 260/240 tetramers bind laterally to the filament. Finally, evidence is presented that indicates that filaments cross‐linked by TW 260/240 are stabilized against shear‐dependent breakage.