Length distribution of F-actin transformed from Mg-polymer.

Abstract

Abstract The length distribution of F-actin produced from Mg-polymer formed in the presence of β-actinin was investigated in detail by electron microscopy. When Mg-polymer was formed at a low MgCl2 concentration (approx. 1 mM), where a fairly large amount of G-actin coexisted with Mg-polymer, and was transformed into F-actin by the addition of KCl at a fairly high temperature (45 °C), the length distribution of F-actin was of a Poisson type. The number average length was approximately 0.6 μm. On the other hand, the formation of Mg-polymer at a high MgCl2 concentration (approx. 2mM), where only a small amount of G-actin remained, resulted in a simple exponential distribution of particle length of the transformed F-actin and its average lengths became shorter. Moreover, when the amount of G-actin coexisting with Mg-polymer was increased by the addition of G-actin to the Mg-polymer solution and transformation then induced, the average length of the resultant F-actin was increased and the length distribution became more homogeneous. These results are explained by the theories of polymerization in systems with a fixed number of polymers. The amount of G-actin coexisting with Mg-polymer also increased with increase of total actin concentration. However, in this case, the average lengths were not significantly changed after its transformation into F-actin. When the transformed F-actin solution was left standing for a long time, the length distribution gradually changed from the Poisson type to the simple exponential type. This was probably due to the fact that a re-arrangement of the preformed F-actin took place. The relaxation time for such redistribution was about 25 h at 35 °C. Both β-actinin and ATP were indispensable for the transformation of Mg-polymer into the F-actin particles having a homogeneous length distribution.