Theoretical study of protein--lipid interactions in bilayer membranes.

An analysis is given for the perturbation of the order and composition of lipid bilayers near an intrinsic membrane protein. Two cases are examined: the protein influences the lipid order (i.e., "fluidity"), and the protein associates with one component of a lipid mixture preferentially. The order perturbation is studied as a function of temperature and lateral pressure by using Landau--de Gennes theory and a variational procedure. It is concluded that, for a given lateral pressure, the greatest amount of boundary lipid is present at the lipid phase-transition temperature. A critical point for the phase transition occurs, near which the amount of boundary lipid increases dramatically. The composition perturbation is modeled in a binary lipid mixture by using a simple regular solution theory. The perturbation is found not to extent much beyond the directly bound layer of lipids unless the solution is near a critical mixing point.