Surface-induced polymerization of actin

Living cells contain a very targe amount of membrane surface area, which po tentially influences the direction, the kinetics, and the localization of b iochemical reactions. This paper quantitatively evaluates the possibility t hat a lipid monolayer can adsorb actin from a nonpolymerizing solution, ind uce its polymerization, and form a 2D network of individual actin filaments in conditions that forbid bulk polymerization. G- and F-actin solutions we re studied beneath saturated Langmuir monolayers containing phosphatidylcho line (PC, neutral) and stearylamine (SA, a positively charged surfactant) a t PC:SA = 3:1 molar ratio. Ellipsometry, tensiometry, shear elastic measure ments, electron microscopy, and dark-field tight microscopy were used to ch aracterize the adsorption kinetics and the interfacial polymerization of ac tin. In all cases studied, actin follows a monoexponential reaction-limited adsorption with similar time constants (similar to 10(3) s). At a longer t ime scale the shear elasticity of the monomeric actin adsorbate increases o nly in the presence of lipids, to a 2D shear elastic modulus of mu approxim ate to 30 mN/m, indicating the formation of a structure coupled to the mono layer. Electron microscopy shows the formation of a 2D network of actin fil aments at the PC:SA surface, and several arguments strongly suggest that th is network is indeed causing the observed elasticity. Adsorption of F-actin to PC:SA leads more quickly to a silghtly more rigid interface with a modu lus of mu approximate to 50 mN/m.