Protein 4.1 and minispectrin-dependent ternary complex formation. (A) Minispectrin-dependent binding of [125I]-protein 4.1 to F-actin. [125I]-Protein 4.1 (0.38μM), F-actin (2.38μM as G-actin), and increasing amounts of either minispectrin or minispectrin-Δ13 were combined, and the bound 4.1R was measured after pelleting the F-actin. (B) Protein 4.1-dependent binding of [125I]-minispectrin or [125I]-minispectrin-Δ13 to F-actin. Increasing amounts of protein 4.1R were added to the [125I]-labeled minispectrins (0.54μM) and F-actin (2.38μM), and each bound minispectrin was measured after pelleting the F-actin. Binding of the mutant minispectrin is impaired under both conditions.