Band 3, membrane-spanning protein

Fig. 10.6. Proteins in the red blood cell membrane. The proteins named Band 3 (the bicarbonate-chloride exchange transporter) and glycophorin contain nonpolar a-helical segments spanning the lipid bilayer. These proteins contain a large number of polar and charged hydrophilic amino acids in the intracellular and extracellular domains. On the inside of the cell, they are attached to peripheral proteins constituting the inner membrane skeleton. Band 3 is connected to spectrin filaments via the protein ankyrin. Glycophorin is connected to short actin filaments and spectrin via protein 4.1.

Fig. 5.4 The OmpF porin from Eschericia coli is an integral membrane channel-forming protein which spans the outer membrane in Cram-negative bacteria. The structure of a monomer of the OmpF porin is shown. In total, 16 /3-bands are configured in the form of a cylinder and form the walls of a pore through which selective passage of ions takes place.

The red cell membrane must be highly deformable to allow it to travel throughout the capillary system in the body. This is because of a complex cytoskeletal structure that consists of the major proteins spectrin, ankyrin, and band 3 protein Mutations in these proteins lead to improper formation of the membrane cytoskeleton, ultimately resulting in malformed red cells, spherocytes, in the circulation. Spherocytes have a shortened life span, leading to loss of blood cells. 

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