F-actin retains a memory of angular order

Modifications can be made to F-actin that do not interfere with the binding of myosin but inhibit force generation, suggesting that actin's internal d ynamics are important for muscle contraction. Observations from electron mi croscopy and x-ray diffraction have shown that subunits in F-actin have a r elatively fixed axial rise but a variable twist. One possible explanation f or this is that the actin subunits randomly exist in different discrete sta tes of "twist," with a significant energy barrier separating these states. This would result in very slow torsional transitions. Paracrystals impose i ncreased order on F-actin filaments by reducing the variability in twist. B y looking at filaments that have recently been dissociated from paracrystal s, we find that F-actin retains a "memory" of its previous environment that persists for many seconds. This would be consistent with slow torsional tr ansitions between discrete states of twist.