Abstract
The intrinsic surface activity of the contractile protein actin has been determined from surface tension measurements using the Wilhelmy hanging‐plate method. Actin, a very soluble protein, moves from the subphase to the air‐water interface to make a film. In the absence of magnesium, actin is monomeric and is known as G‐actin. During the compression the monomers change their conformation or orientation at the interface and they are then pushed reversibly into the subphase upon further compression. No collapse occurs. Actin monomers in the presence of magnesium become activated; at concentrations greater than some critical value, actin polymerizes to form filaments of F‐actin. The actin filaments have a higher surface activity than the actin monomers either because they are more hydrophobic or because F‐actin, a rigid polymer, is much more efficient at creating excluded volume. The actin filaments then form a rigid film at the interface that collapses when the surface area is decreased. At less than the critical concentration, the actin monomers are present in the subphase in their activated form. However, their concentration increases at the interface during film compression until the critical concentration is reached. The surface pressure isotherm in this case has the characteristics of a G‐actin film at the beginning of the compression and of an F‐actin film at the end of the compression process. © 2003 Wiley Periodicals, Inc. Biopolymers 70: 289–296, 2003