The influence of substoichiometric concentrations of myosin subfragment 1 on the state of aggregation of actin under depolymerizing conditions

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Abstract
In 3 mM KCl, 2 mM Tris/HCl pH 7.5, 22.degree. C, 0.38 .mu.M myosin subfragment 1 delays the depolymerization of F-actin (7.2 .mu.M measured as monomer). The depolymerization proceeds rapidly for a few minutes and then slows down suddenly when the ratio between the monomers in the actin filaments and myosin subfragment 1 reaches the value of 11. At this time myosin subfragment 1 is substantially all bound to the actin polymers which form an irregular and discontinuous network of filaments running in doublets and in triplets, perhaps cross-linked by myosin subfragment 1. Depolymerization proceeds then for several hours, apparently ending up with the formation of the 1:1 actin-S1 heteropolymer. The ratio between the monomers in the actin filaments and myosin subfragment 1 at the end of the rapid depolymerization process is different for different proteins preparations and may be as low as 5.5. In 2 mM Tris/HCl pH 7.5, 25.degree. C, 1 .mu.M myosin subfragment 1 is able to induce the formation of undecorated actin filaments from 12 .mu.M ATP-G-actin. These filaments probably originate by redistribution of myosin subfragment 1 between the newly formed 1/1 actin-S1 heteropolymer and G-actin in the medium, a process which allows the transient formation of undecorated actin filaments.