Chapter 3 Lipid-protein interactions in biological membranes

Summary Lipids and proteins are the main components of any biological membrane. Whereas the fluid-like lipid bilayer determines the basic structure of membranes, proteins are responsible for most membrane functions. Like the lipid molecules, most membrane proteins are mobile in the plane of the membrane, and both are distributed asymmetrically within the bilayer. Membrane proteins interact with membranes by a variety of different means. Many membrane proteins extend across the lipid bilayer. Others do not span the bilayer but instead are attached to the one or the other side of the membrane. Some membrane proteins have nonpolar domains which interact with the hydrophobic core of the bilayer. Others are associated with the surface of the membrane by a combination of electrostatic and hydrophobic noncovalent interactions. Amphitropic proteins are considered to bind reversibly to membranes in response to a physiological signal. Lipids provide a specific microenvironment for many membrane proteins. They are involved in the organization, activation, modulation and regulation of membrane proteins. At any instant in time, a substantial fraction of the lipids in a biological membrane are adjacent to proteins. The layer of lipids surrounding a protein is called the boundary layer or the lipid annulus. Some membrane proteins function only when embedded in specific lipid domains. The lipids surrounding a particular membrane protein may not necessarily have the same composition as the bulk membrane. Proteins as well as lipids can be immobilized and confined to particular domains in a continuous lipid bilayer. The formation of microdomains within membranes, which involves specific lipid-lipid, lipid-protein, and protein-protein interactions, is particularly crucial for understanding the structure and function of biological membranes. At present, biological membranes are best described as a dynamic complex of discrete, interacting microdomains composed of ordered lipid and protein structures, which are in motion with respect to one another within the less organized, fluid-like, bulk lipid bilayer.