Multiple supramolecular structures formed by interaction of actin with protamine.

When protamine is added to actin, different supramolecular structures are formed depending on the molar ratio of the two proteins and of the ionic strength of the medium. At low ionic strength, and going from a molar ratio of protamine to G-actin of 4:1, 2:1 and 1:1, globular aggregates are first converted into extended structures and then to long threads in which the constituent ATP-G-actin is rapidly exchangeable with the actin of the medium. At high ionic strength {Tyrode [(1910) Arch. Int. Pharmacodyn. Ther.20, 205-212] solution}, starting from G-actin and protamine in the 1:1 molar ratio, long ropes are formed that can be resolved into intertwining filaments of 4-5nm diameter. The addition of protamine in a 1:1 molar ratio to a solution of F-actin in Tyrode solution causes the breakage of the actin filaments, which is also revealed by the decrease of the viscosity of the solution and the formation of ordered latero-lateral aggregates. The structures formed by reaction of protamine with G-actin can be separated from free G-actin and protamine by filtration through 0.45mum-pore-size Millipore filters. This technique has been exploited to study the exchange reaction between free actin and the actin-protamine complexes. For these studies the 1:1 actin-protamine complex formed at low ionic strength and the 2:1 actin-protamine complex formed in the presence of 23nm-free Mg(2+) have been selected. In the first case the exchange reaction is practically complete in the dead time of the experiment (20s). In the second case, where the complex operates like a true ATPase, the rate of the exchange is initially comparable with the rate of the ATP cleavage. Later on, however, the complex undergoes a change and the rate of the exchange between free actin and the actin bound to protamine becomes lower than the rate of the ATPase reaction. It is proposed that the ATP exchanges for ADP directly on the G-actin bound in the complex.