ABSTRACT
Biological membranes are fundamentally asymmetric. The integral proteins have a preferred transmembrane orientation, presumably reflecting the initial biosynthesis and membrane insertion mechanism. The preferred type of association for homopolymeric membrane proteins is that of cyclic symmetry, with the axis of rotational symmetry passing normally through the membranes. This type of association preserves the transmembrane orientation of the subunits and provides maximum stabilization through protein-protein interactions, as well as through interactions between the proteins, lipids, and aqueous environment. However, many of the proteins of biomembranes do not float independently in an indifferent lipid bilayer. Rather, there exist complex and specific interactions between proteins. Although the lipids of biological membranes provide a permeability barrier and a fluid boundary to the cell, the membrane proteins are responsible for specific recognition phenomena, enzymic activities, specific transport, transmembrane signaling, and structural elements that contribute to the morphology of the cell.
