The nature of the globular-to fibrous-actin transition

Actin plays crucial parts in cell motility through a dynamic process driven by polymerization and depolymerization, that is, the globular ( G) to fibrous ( F) actin transition. Although our knowledge about the actin- based cellular functions and the molecules that regulate the G- to F- actin transition is growing, the structural aspects of the transition remain enigmatic. We created a model of F- actin using X- ray fibre diffraction intensities obtained from well oriented sols of rabbit skeletal muscle F- actin to 3.3 A in the radial direction and 5.6 A along the equator. Here we show that the G- to F- actin conformational transition is a simple relative rotation of the two major domains by about 20 degrees. As a result of the domain rotation, the actin molecule in the filament is flat. The flat form is essential for the formation of stable, helical F- actin. Our F- actin structure model provides the basis for understanding actin polymerization as well as its molecular interactions with actin- binding proteins.