Cell Surface Properties Revealed by Erythrocytes (With Diagram)

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.