Lipid monolayers: Mechanisms of protein penetration with regard to membrane models

The influence of lipid and protein on the properties of the air-water interface is analyzed with the view to formulate a mechanism of interaction of protein with lipid monolayers. The increase in surface pressure (ΔΠ) and the quantity of protein incorporated in the lipid film after injection of protein under lipid monolayers were studied as a function of both lipid structure and protein structure. With rabbit γ-globulin, the values of ΔΠ were cholesterol > phosphatidyl choline > sphingomyelin. Similar results were obtained with ribonuclease, lysozyme and serum albumin. The quantities of protein found in films of either cholesterol or phosphatidyl choline (egg lecithin) were much larger than those calculated from a geometric model in which a protein monolayer occupies the area made available by the compressed lipid. Arguments are produced against penetration based on simple mechanisms of compressibility of the lipid film. The mechanisms operating in the incorporation of protein into lipid monolayers are grouped into three categories: (a) free penetration, typical of lecithin; (b) binding-mediated penetration, typical of cholesterol and some glycosphingolipids; and (c) binding-inhibited penetration, typical of the albumin-ganglioside system and a specific lipid hapten-antibody system. A model is described in which nonspecific protein interacts with polymeric lecithin structures (surface micelles). In the sequence of events X»Y»Z, the globular protein X is activated into the expanded or extended form Y by contact with the lipid and then restructured into a compact form Z with release of water and free energy. The resulting lipid-protein assembly has a mosaic structure in which lipid and protein polar surfaces are exposed to water. Accessibility of lecithin to phospholipase A is consistent with the model and with current views on the state of protein in biological membranes; according to such views, protein is more likely structured inside the lipid milieu and not simply denatured on the lipid-water interface.