Theory of protein-lipid and protein-protein interactions in bilayer membranes.

A model for protein-lipid interactions in bilayer membranes where the proteins are very dilute is extended to higher protein concentration, where appreciable lipid-mediated protein-protein interactions occur. It is found that proteins may change the lipid phase transition temperature and that they weaken the phase transition. There exists a critical protein concentration above which the sharp lipid phase transition is abolished. The model also qualitatively reproduces several experimental observations on the physical behavior of bilayers formed from mixtures of cholesterol and phosphatidylcholines.