In striated muscle the pointed ends of polar actin filaments are directed toward the center of the sarcomer. Formed filaments keep a constant length of about 1 pm. As polymerization and depolymerization at free pointed ends are not sufficiently slow to account for the constant length of the filaments, we searched for proteins which occur in sarcomers and can stabilize the pointed ends of actin filaments. We observed that tro~myosin-troponin complex reduces the rate of association and dissociation of actin molecules at the pointed ends more than 30-fold. On the average, every 600 s one association or dissociation reaction has been found to occur at the pointed ends near the critical actin monomer concentration.

The rate constant and equilibrium constant of association of an actin monomer with 1:1 gelsolinactin complex isolated from chicken were measured by using fluorescently labeled actin. According to fluorescence stopped‐flow experiments, the rate constant of formation of the 1:2 gelsolin‐actin complex from 1:1 gelsolin‐actin complex and actin was found to be about 2 × 107 M‐1 s‐1 under conditions where gelsolin binds Ca2+. The rate of dissociation of one actin molecule from the 1:2 gelsolin‐actin complex was determined by exchange of actin for fluorescently labeled actin. The rate constant of dissociation was about 0.02 s‐1. Thus, the equilibrium constant for association of actin with 1:1 gelsolin‐actin complex can be calculated to be in the range of 109 M‐1. The rate of dissociation of actin from 1:2 gelsolin‐actin complex was independent of the Ca2+ concentration. Ca2+ affects only the rate of association of actin with 1:1 gelsolin‐actin complex.

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