Glycoproteins - Hughes R. 1985

Structure
Membrane glycoproteins. General concepts
Erythrocyte membranes

Since Glycoproteins and simple Proteins can integrate into membranes, it is necessary to explain how a polypeptide consisting of many Amino Acids with charged side chains interacts with a lipid-based Structure. Most likely, proteins employ the same principle as Lipids, which are amphipathic molecules containing a polar, often charged group and a hydrophobic fatty acid tail. Lipids are arranged within the membrane bilayer such that their polar groups face outward, while the fatty acid chains point inward, forming an impermeable hydrophobic layer. Membrane-associated Polypeptides, such as Glycophorin, feature chain segments with nonpolar side groups that span the membrane itself. Evidence suggests that the carbohydrate-rich domain of glycophorin A is located on the outer surface of the erythrocyte, whereas the carboxy-terminal segment of the chain, containing an Abundance of acidic amino acids, projects into the Cytoplasm (Fig. 2.20). Several other membrane glycoproteins are known to have a similar Organization. However, data obtained from studying the second major component of human erythrocyte membranes urge caution regarding the assumption that all glycoproteins conform to such a simple model. The so-called "band 3" protein is involved in anion transport across human erythrocyte membranes. The complete Amino Acid Sequence of this protein has not yet been determined, but it has been established that its polypeptide chain traverses Cell/29.html">The Lipid Bilayer not once, but multiple times (Fig. 2.20). The amino-terminal region of the chain extends into the cytoplasm, while the carboxy-terminal region, bearing glycans, is exposed on the outer surface of The Cell. Nevertheless, the fundamental principle established earlier for glycophorin holds true here as well—specific Regions of the protein reside outside the Hydrophobic core of the membrane. These extracellular segments of the polypeptide chain are believed to interact with other proteins. For instance, there is compelling evidence that the globular protein ankyrin binds selectively to the amino-terminal portion of the band 3 protein. In turn, ankyrin apparently interacts with spectrin, a fibrillar protein of the erythrocyte cytoplasm. A spectrin lattice lines the inner surface of The erythrocyte membrane, imparting structural rigidity and maintaining the characteristic biconcave shape of the erythrocyte. The presence of Actin on the cytoplasmic side of the membrane is also likely crucial for preserving this cell shape. Spectrin presumably binds to actin or its tetramer at a site distinct from the ankyrin-binding region. Actin itself is likely anchored to the membrane via yet another erythrocyte protein, the so-called "band 4.1" protein. The latter exhibits a certain affinity for the polar side groups of Phospholipids and may thereby facilitate the interaction of actin tetramers with the inner leaflet of the erythrocyte membrane [21].

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The findings outlined above help us understand how a relatively simply organized membrane can acquire enhanced rigidity through the involvement of muscular apparatus components—a property essential for the cell to maintain an unusual, energy-demanding shape. It is quite evident how crucial this is for preserving specific cell Morphology and motility [22]. Some of these issues are briefly discussed in the final chapter.