Vinculin-lipid monolayer interactions: a model for focal contact formation

Vinculin-lipid interactions were investigated in a modified Langmuir trough. Provided proper conditions, vinculin had the potential to penetrate into phospholipid monolayers and to form rigid, cohesive protein films even at phospholipid monolayer pressures similar to those assumed to exist in living cell membranes. The equilibrium constant for this reaction was estimated to be on the order of 2.5 X 10(-9) to 2.2 X 10(-7) mol/liter (for pressures between 25 and 35 mN/m). Penetration velocity depended on lipid composition: it was high with acidic phospholipids, intermediate with mixtures of acidic and neutral phospholipids, and low with neutral phospholipids. Electron microscopy of freeze-dried/metal-shadowed vinculin films, recovered from the phospholipid monolayer surface, revealed relatively tightly packed globular particles, 13 to 18 nm in diameter, on average significantly larger than the particles seen in glycerol-sprayed and rotary metal-shadowed preparations of soluble vinculin. The lipid monolayer penetration ability of vinculin appears to depend on its conformation. Acid treatment or low salt buffers induced reversible changes in vinculin conformation such that it abolished its lipid penetration potential. These conformational changes could be documented by both circular dichroism and fluorescence spectroscopy. These results indicate that in the focal contact area vinculin may act like a "glue" and link, in a reversible way, stress fibers of cultured cells via their anchor proteins to the extracellular matrix.