Figure 3.

The double-stranded form is important in defining the polarity of dynamics. A simplified model to explain the difference in the dynamics between the two ends is presented. A more realistic and complicated model was proposed previously;⁵ although the concept is the same. A: A simplified illustration of the actin monomer. The switching loop is the DNase-I binding loop in the simplified model. The actin monomer has two binding sites for the switching loop (cyan circles). One is a strong binding site (the large circle) and the other is a relatively weak binding site (the small circle). B: A model of the barbed end. The switching loop of the end subunit (in red) binds to the strong binding site on the actin subunit in the same strand. As a result, a strong binding site for an incoming actin monomer is available and the incoming monomer can easily bind to the end. C: A model of the pointed end. The switching loop of the adjacent subunit at the end (in red) binds to the weak binding site on the end subunit in the other strand. Consequently, the dissociation of the end subunit becomes more difficult at the pointed end because the extra binding by the switching loop (in red) prevents the end subunit from dissociating. When a new actin monomer comes in close proximity to bind to the strand, the switching loop (in red) that is already bound to a different site must first dissociate from the weaker site prior to interacting with strong binding site of the new monomer. As a result, the depolymerization and polymerization rates at the pointed end are slower than at the barbed end and represents the origin of the polarity of the dynamics of the actin filament. D-F: When the filament is single-stranded, it is difficult to define the polarity of the dynamics. If we assume a similar switching loop in the single-stranded filament, the weak binding site for the switching loop to bind at the pointed end must be located in the same strand because there is only one strand present. However, the switching loop in the monomer can also bind to the weak binding site in the same monomer (D). In this case, the polymerization at the pointed end (E) and at the barbed end (F) is inhibited to the same extent because of the switching loop binding to the weak binding site. For the dissociation from the ends, the situation cannot be different because the connection to be severed for dissociation to occur is identical between the two ends. Therefore, the rates of polymerization and depolymerization at the both ends must be similar when the filament is single-stranded.