**2. Erythrocyte membrane protein composition**

Erythrocyte membrane proteins can be classified into three categories, according to their functional properties in the membrane struture (An & Mohandas, 2008; Mohandas & Gallagher, 2008). The first includes cytoskeletal proteins, as spectrin (α and β chains), protein 4.1, and actin; the second includes integral/transmembrane proteins of which the representative proteins are band 3 and glycophorins; the third includes anchoring/linker proteins, namely, ankyrin (also known as band 2.1) and protein 4.2. The anchoring/linker membrane proteins mediate the attachment of cytoskeletal proteins to integral proteins (Fig. 1) (Lucchi, 2000; Gallagher, 2005; Mohandas & Gallagher, 2008).

Fig. 1. Schematic representation of red blood cell membrane, showing the topographical localization of proteins and their interactions. The membrane is a complex structure in which a plasma membrane envelope composed of amphiphilic lipid molecules is anchored to a two dimensional elastic network of skeletal proteins through tethering sites (transmembrane proteins) embedded in the lipid bilayer. Adapted from An & Mohandas, 2008.

The cytoskeleton is a 3-dimensional network of proteins that covers the cytoplasmatic surface of the erythrocyte membrane and is responsible for its biconcave shape and the properties of elasticity and flexibility. It comprises approximately half the membrane protein mass and is primarily composed of spectrin, protein 4.2 and actin. Spectrin is the major protein of the cytoskeleton, and, therefore, the primary cause of erythrocyte shape, integrity and deformability. It is linked to the lipid bilayer, by vertical protein interactions with the transmembrane proteins, band 3 and glicophorin A (Lucchi, 2000). In the vertical protein interaction of spectrin with band 3 there are also ankyrin (also known as band 2.1) and protein 4.2 involved. A normal linkage of spectrin with the other proteins of the cytoskeleton assures normal horizontal protein interactions. The vertical and horizontal interactions between membrane constituents account for the integrity, strength, and deformability of the cell (An & Mohandas, 2008; Mohandas & Gallagher, 2008). Disruption of vertical interactions because of membrane protein deficiencies favours membrane vesiculation with loss of surface area and development of spherocytic cells, with increasing rigidity of the cell membrane that may lead to premature spleen sequestration and destruction (An & Mohandas, 2008).
