Erythrocyte Membranes

Erythrocytes (red blood cells) can be easily lysed to release their contents and yield membrane ghosts. These are large vesicles that represent a nearly pure preparation of the plasma membrane of the cells. The lipid composition and distribution between the inner and outer leaflets of the bilayer membrane are shown in Table 10.3 and Figure 10.15, respectively.

Peripheral membrane proteins can readily be removed by changes in ionic strength or pH. Extraction of the membrane ghosts with detergent removes integral membrane proteins that are not part of the membrane skeleton and leaves behind only the membrane skeleton proteins. The proteins of the intact ghost and membrane skeleton only are shown in Figure 10.17.

The primary integral proteins of ghosts are bands 3, band 4.5, and glycophorins A, B, and C (Table 10.5). Band 3 protein helps to anchor the membrane to the membrane skeleton through interaction with the ankyrin protein.

> Band 3 Protein - the most abundant erythrocyte membrane protein. It is an anion channel which facilitates the exchange of HCO 3-for Cl-.

> Glycophorins - a group of integral membrane proteins with a variety of functions. They are covalently linked to external, carbohydrate domains that are linked to sialic acid residues.

The membrane skeleton is responsible for the shape of the membrane ghost. A schematic structure of the protein is shown in Figure 10.18. The skeleton is a two-dimensional network of some of the peripheral membrane proteins. The roles of some of these are as follows:

> Spectrin - elongated molecules made up of 2 2 tetramers. The tetramers form a chain-like structure that links the many components of the ghost and the skeleton. Spectrins contain a large portion of their structure in the form of an -helix and appear to be linked at their ends through short chains of actin molecules, together with band 4.1 protein and adducin.

> Ankyrin - anchors the membrane to the skeleton by linking with spectrin and band 3 protein.

> Band 4.1 - also helps anchor the membrane to the skeleton by linking spectrin to glycophorin.

See also: Membrane Asymmetry, Membrane Proteins