F-actin is a helix with a random variable twist.
E H Egelman, N Francis, D J DeRosierAbstract
Image analysis of isolated F-actin filaments shows that the actin helix can be described by a constant rise per subunit but a considerably variable and randomized twist (number of units per turn). The ability of actin subunits to rotate through angles of the order of 10 degrees from their helically ideal positions helps to explain actin's capacity to form many different polymorphic structures.