Lateral and Transverse Asymmetry in Membranes

Abstract

All biomembranes are asymmetric, and it is easy to rationalize why this should be so, since all membranes have two faces, exposed to different environments, e.g., cytoplasmic and extracytoplasmic. Therefore, transverse asymmetry, differentiating the two monolayer halves of the bilayer, is sensible. Membrane protein asymmetry is clearly a consequence of the way in which the proteins are originally inserted into the membrane. The rate of protein flip-flop across the bilayer is negligible. The term “flip-flop” is generally used to denote a rotation of 180 degrees about an axis in the plane of the membrane. Membrane lipids are also asymmetrically disposed, most convincingly demonstrated in the case of the red blood cell. How lipid transverse asymmetry originates and is maintained is not entirely clear at this time. Physical forces, such as those caused by extreme membrane curvature, may be important in some cases, but cytoskeletal interactions and possibly ATP-requiring enzyme “flipases” may be generally important.