Model for VASP barbed end association. In the absence of monomeric actin (right side), the GAB and FAB domains of VASP contribute to actin filament binding. VASP binds with equal affinity along the sides and near the ends of the filament. Apparent barbed end association of VASP alone is indistinguishable from lateral interaction with F-actin. When the GAB domain is bound to monomeric actin (left side), binding to the sides of filaments is weakened and the bound actin monomer allows VASP to capture an actin filament barbed end. Note: although the mechanism described occurs in the context of a VASP tetramer, a single subunit is drawn for simplicity.