The surface properties of lung 36 kDa Ca(2+)-dependent phospholipid-binding protein

The intrinsic surface activity of a 36 kDa rabbit lung calcium-dependent phospholipid-binding protein (PLBP), a member of the annexin family of such proteins, at the air/water interface has been determined from measurements of surface tension of aqueous solutions, and surface concentration of 14C-labeled PLBP adsorbed from aqueous solution in the absence and presence of Ca2+. It was also possible to spread insoluble monolayers of PLBP to determine surface pressure vs. surface concentration isotherms, as well as surface elasticity and surface viscosity as a function of frequency from electrocapillary wave diffraction measurements. PLBP has been shown to exhibit significant intrinsic surface activity at the air/water interface, comparable to a variety of other hydrophobic proteins known to be quite surface active. In all cases, surface properties were enhanced by the presence of Ca2+, particularly the degree of surface viscoelasticity at close-packing in the monolayer. This is believed to reflect changes in protein conformation at the surface.