Polyphosphoinositides inhibit the interaction of vinculin with actin filaments

Binding of vinculin to adhesion plaque proteins is restricted by an intramo lecular association of vinculin's head and tail regions. Results of previou s work suggest that polyphosphoinositides disrupt this interaction and ther eby promote binding of vinculin to both talin and actin. However, data pres ented here show that phosphatidylinositol 4,5-bisphosphate (PI4,5P(2)) inhi bits the interaction of purified tail domain with F-actin. Upon re-examinin g the effect of PI4,5P(2) on the actin and talin-binding activities of inta ct vinculin, we find that when the experimental design controls for the eff ect of magnesium on aggregation of PI4,5P(2) micelles, polyphosphoinositide s promote interactions with the talin-binding domain, but block interaction s of the actin-binding domain. In contrast, if vinculin is trapped in an op en confirmation by a peptide specific for the talin-binding domain of vincu lin, actin binding is allowed. These results demonstrate that activation of the actin-binding activity of vinculin requires steps other than or in add ition to the binding of PI4,5P(2).