F-Actin

The classical description of F-actin states that it has a filamentous structure that can be considered to be a single stranded levorotatory helix with a rotation of 166° around the helical axis and an axial translation of 27.5 Å, or a single stranded dextrorotatory helix with a cross over spacing of 350-380 Å, with each actin surrounded by four more. The symmetry of the actin polymer at 2.17 subunits per turn of a helix is incompatible with the formation of crystals, which is only possible with a symmetry of exactly 2, 3, 4 or 6 subunits per turn. Therefore, models have to be constructed that explain these anomalies using data from electron microscopy, cryo-electron microscopy, crystallization of dimers in different positions and diffraction of X-rays. It should be pointed out that it is not correct to talk of a “structure” for a molecule as dynamic as the actin filament. In reality we talk of distinct structural states, in these the measurement of axial translation remains constant at 27.5 Å while the subunit rotation data shows considerable variability, with displacements of up to 10% from its optimum position commonly seen. Some proteins, such as cofilin appear to increase the angle of turn, but again this could be interpreted as the establishment of different "structural states". These could be important in the polymerization process.