IQGAP1 promotes the dynamic exchange of actin filament end-binding proteins. (A–C) A representative seeded actin assembly assay showing the influence of IQGAP1 on actin assembly in the presence of (A) formin (mDia1(∆DAD)) (B) Capping Protein (CP), and (C) formin-CP “decision complexes.” Assays contain prepolymerized (unlabeled) actin seeds, 0.5 µM actin monomers (5% pyrene-labeled), 5 µM PFN1 and 75 nM IQGAP1, 2 nM mDia1(∆DAD), or 10 nM CP, as noted. Curves in A–C were plotted for clarity and were generated from the same read on a plate reader (n = 3 total were performed). (D) Kymographs from four-color TIRF movies of stabilized actin filament seeds show the status of end binding proteins (EBPs) on actin filament plus ends. Individual channels, EBP merge (without actin seed), and the merge of all four wavelengths are shown. Reactions contain actin filament seeds (10% Dylight 405 label stabilized with 132 nM Alexa 405 phalloidin) and various combinations of 10 nM 549-SNAP-mDia1(ΔDAD), 10 nM 649-SNAP-Capping Protein (CP), and 10 nM 488-SNAP-IQGAP1. Scale: length, 1 µm; time, 30 s. (E) Survival plots of the plus-end occupancy of indicated proteins from reactions as in D (n = 10–48 molecules per condition, as stated, from n = 3 independent reactions). (F) Summary of IQGAP1 displacement activities at filament plus ends.