In view of the fact that the mature erythrocyte lacks organelles, this cell has always been a popular source of plasma membranes. Indeed, the pioneering studies of Gorter and Grendel, which were the first to indicate the existence of the lipid bilayer, were carried out using erythrocytes.
Like the plasma membranes of other cells, the red blood cell membrane is asymmetric. The lipid asymmetry of the erythrocyte membrane has already been described (Table 15-3).
Regarding protein asymmetry, the peripheral proteins account for about 40% of all membrane proteins but are restricted to the membrane’s interior surface.
The most abundant of these proteins and the first to be isolated is spectrin. Spectrin is a fibrous protein consisting of two large polypeptide chains and having a length of about 200 nm. This protein is believed to be an important component of a weblike network of proteins on the interior membrane surface.
Spectrin molecules are not attached directly to the membrane’s inner surface rather they are anchored to the membrane via other membrane proteins including ankyrin and actin. The spectrin-ankyrin-actin complexes create a web-like network or cytoskeleton that supports the membrane and contributes to the biconcave shape that characterizes mammalian red cells. Spectrin may not be limited to erythrocytes as spectrin or at least spectrinlike proteins have recently been isolated from the plasma membranes of other cells.
The erythrocyte membrane possesses two major integral proteins that span the lipid bilayer. One of these, called glycophorin-A, has been fully sequenced and reveals several very interesting properties. Glycophorin-A consists of a single chain of 131 amino acids; 16 short carbohydrate chains are linked to residues near the N-terminus of the polypeptide (primarily to serine and threonine side chains), the carbohydrate accounting for about 60% of the total mass of the glycoprotein.
The N-terminal region of glycophorin-A is thought to project beyond the exterior membrane surface, the last five amino acids determining the MN blood group status of an individual. The C- terminal end of glycophorin-A is rich in acidic amino acids, especially glutamic acid, and is believed to project into the cell interior. A segment of about 20 amino acids in the middle of the polypeptide consists exclusively of nonpolar and hydrophobic amino acids and apparently is that portion of glycophorin-A that spans the lipid bilayer.
In addition to its supportive role, a cell’s cytoskeleton acts to restrict the lateral movement of proteins within the membrane. In red cells, the spectrin- ankyrin-actin complexes place constraints on the mobility of glycophorin-A. Antibodies against spectrin cause aggregation of spectrin molecules and their precipitation onto the inner surface of the erythrocyte membrane, and this is accompanied by a corresponding rearrangement of glycophorin-A in the membrane.
In nucleated cells, the cytoskeleton is more extensive and includes myosin filaments and microtubules as well as actin and other proteins (Fig. 15-21). The network of cytoplasmic filaments and microtubules radiates through much of the cytosol and provides points of attachment for many of the cellular organelles.
Whereas the rearrangement of cytoskeletal components just below the cell surface manifests itself in the redistribution of integral membrane proteins, major movements of the cytoskeleton may be fundamental to such gross activities as cellular motion and endocytosis and exocytosis.
The differentiation of the erythrocyte in the bone marrow is accompanied by a major reorganization of the cell in which organelles like the nucleus, mitochondria, intracellular membranes, ribosomes, and so forth are progressively lost. Despite the lack of major internal structures and its seeming simplicity, the erythrocyte retains a characteristic shape. In humans (and in most other mammals), the cell takes the form of a biconcave disk with a diameter of about 8 µm (Fig. 15-22), although changes in shape are readily induced by variations in osmotic pressure.
The biconcave shape of the erythrocyte is important in its biological function, because such a shape maximizes oxygen diffusion from the cell into the tissues and promotes efficient stacking of cells (rouleaux formation) as the red cells circulate through the narrow capillary passageways. The biconcave shape of the red cell is often deformed as it circulates through the narrowest capillaries, but the- shape is quickly restored in the larger passageways.
As noted above, the characteristic red cell shape is maintained by the cytoskeletal protein network that lies just below the membrane surface. The importance of cytoskeletal support is documented by the observation that the shape of the cell is maintained even when nonionic detergents are used to extract the membrane’s lipid bilayer and its intrinsic proteins.
In a series of rather startling experiments, B. Bull and J. D. Brailsford have shown that when an erythrocyte is attached by a portion of its undersurface to a glass slide and a laser used to make a visible mark on the membrane’s surface so that membrane movement can be followed, slight lateral displacement of the cell using hydraulic force is accompanied by the membrane rolling in the direction of the force, much like a tank track does (Fig. 15-22).
The laser mark travels over the cell surface, following the contours of the biconcave shape. In other words, though rolling laterally, the biconcave shape of the cell is maintained with the vertical biconcavity moving parallel to the glass surface. Still unanswered is whether the membrane is being displaced relative to the underlying cytoskeleton.
Erythrocytes traveling through capillaries and other blood vessels are often arranged as stacks or rouleaux, with their biconcave faces juxtaposed. Such an orderly procession makes it possible for so large a number of cells to pass through the body tissues in short periods of time. P. B. Canham has shown that when a cell in rouleaux is struck by a laser beam of sufficient energy, the cell membrane is disrupted and the cell lyses (bursts).
However, several cells in the rouleaux on either side of the target cell (and not directly affected by the laser) also are seen to undergo gradual lysis. This “contagious lysis” apparently results from the fact that the membranes of erythrocytes in rouleaux transiently adhere to one another.
Sudden movement of the membranes of one cell (as during lysis) creates sufficient shear forces at contact points with neighboring cells that their membranes are also affected. The nature of membrane interaction between neighboring erythrocytes in rouleaux is unknown, but it is clear that much remains to be learned about the “simple” red blood cell.