Reaction scheme of actin binding and exchange by Tβ4 and profilin. Species with significant and insignificant contributions to barbed-end elongation are colored green and magenta, respectively. Numbers represent thermodynamic equilibrium constants that are either measured directly [blue, constants in this paper; orange, constants reported by Yarmola and Bubb (9)] or calculated through energy balances (black). A, ATP–G-actin; P, profilin; T, Tβ4; Tn, Tβ4N.