Fig. 3.

Graphical depiction of actin complexed with ABPs. (a) Actin filaments can be reinforced mechanically by interstrand (blue) or intrastrand (red) binding proteins. (b) Tropomyosin, an intrastrand binding protein, binds along the actin filament in the blocked (red) and open (orange) positions. (c) Caldesmon C-terminal fragment H32K is an interstrand binding protein that acts as a molecular staple between actin strands. Under the conditions employed here H32K binding will saturate actin filament binding sites (not shown for image clarity). Phosphorylation (P) of H32K causes one side of the staple to release from the actin, while still remaining bound to actin via the other side of the staple [Foster et al., 2004].