The carboxy-terminal tail domain of vinculin contains a cryptic binding site for acidic phospholipids

Using a gel filtration assay, we have characterized the binding of acidic phospholipids to vinculin. Vinculin binds phosphatidyinositol (K_d ∼ 5 μM) in a reversible, saturable manner in low ionic strength buffers. This interaction is inhibited substantially at 100 mM NaCl and therefore may not be of physiological interest. In contrast, the carboxy-terminal 30-kDa fragment of vinculin, produced by S. aureus V8 protease cleavage, binds acidic phospholipids more tightly than the intact protein, and in a manner insensitive to 100 mM NaCl. Re-addition of the 95-kDa head fragment to the tail restores salt-sensitivity to the tail-lipid interaction. These data indicate that under physiologic ionic conditions, the intramolecular head-tail interaction in vinculin masks a high affinity acidic phospholipid binding site present in the tail domain.