Some Properties of Two Proteins Involved in Membrane Transport

Abstract

One of the most intensively investigated fields of biological research nowadays is membrane structure and function.The reason for this great interest resides in the great number of biological functions connected with these microscopic structures. It is not appropriate to recall here all the concepts which have developed in this area since some decades (see for a recent review 1). The most generally accepted structure today is that illustrated by Singer (2) and referred to as fluid-mosaic model. In the model phospholipids are arranged in a bilayer system,with the hydrophobic tails in close association to each other,whereas the polar heads are facing the water phase on both sides of the membrane. Associated to the phospholipid bilayer are the protein molecules which are to be classified in peripheral and integral ones. The type of interaction of the two classes of proteins with phospholipids are totally different. In the former case protein are bound to the surface via polar groups. Changes in ionic strength and/or chelation of divalent cations is often sufficient to cause the peripheral proteins to be released. On the contrary,integral proteins are firmly bound to the lipid bilayer and hydrophobic interactions contribute greatly to the stabilization of the structure. Obviously depending on the physical properties of the surface of the protein three possibilities are open: a) a protein molecule is completely embedded in the lipid bilayer; b) a protein molecule is only partially embedded in hydrophobic core of the membrane and faces one of the two surfaces with a hydrophilic tail and c) a protein molecule may be so arranged that only the intermediate portion of it shows a hydrophobic outer surface and the protein may be visualized as a transmembrane component.