The Structure of Actin

The protein actin exists as both a monomer (G-actin) and polymer (F-actin). There have been more than 80 different crystal structures of G-actin and related homologs over the years, and at least 4 different structures/models for F-actin have been proposed using a combination of techniques. Comparison of the G-actin crystal structure with the most recent F-actin structures suggests that there are several structural rearrangements, the most significant being a relative rotation of subdomains that results in a propeller-like twist of the protein. To investigate these structures in more detail, we performed multi-microsecond molecular dynamics simulations of the muscle ADP-actin monomer (pdb 1J6Z) and the two most recent ADP F-actin structures (2ZWH and 3MFP). We find that G-actin monomer deviates quite far from its crystal structure over the course of its simulation while the F-actin protomer shows a more modest shift from its starting point. The most interesting result is that the equilibrium structures of the monomer and F-actin protomer are much more similar than previously thought and their conformational spaces show significant overlap. We will present detailed analysis of the conformational dynamics, as well as analysis of the filament conformation, twist, and structures of the barbed and pointed ends.