Uni-directional propagation of structural changes in actin filaments

When a protein molecule is bound with another, its structure is likely to change in one way or the other. The structure of a protein molecule in a protein complex is also likely to change when binding partner in the complex undergoes a conformational change. It is therefore no surprise that binding of an actin-binding protein to a protomer in an actin filament changes the structure of that actin pro-tomer, and that the resultant conformational change in the actin protomer affects the structure of the neighboring protomers in the same filament. Moreover, eukaryotic actin appears to have evolved to efficiently spread the conformational change in the actin protomer initially bound with actin-binding protein over a long distance along the filament (cooperative conformational change), as has been observed in the cases of cofilin- and myosin-induced cooperative conformational changes. We speculate that the high degree of cooperativity in conformational changes in actin filaments enables cooperative binding of actin-binding proteins, which is necessary for actin filaments to perform specific functions by selectively interacting with a subset of actin-binding proteins among the large number of actin-binding proteins present in the cell. Interestingly, cooperative conformational changes propagate to only one direction along the filament, at least in the cases of cofilin and myosin II-induced conformational changes. Functional significance of those uni-directional conformational changes in actin filaments is not known, but we propose that they play roles in directional signal transmission along one-dimensional polymer in cells, or in force generation by myosin.