Spectrin (2SPC) is the major protein component of the "membrane cytoskeleton" in erythrocytes (red blood cells). In the illustration below, one can see two spectrin polypeptides from Drosophila (blue and green in the upper view) folded together into a structure of extended alpha helices (pink in the lower view). Two of these heterodimers then join end-to-end to form a tetramer having a length of about 200 nm. Since erythrocytes are about 5 micrometers across, it would take only 25 tetramers to span the entire cell. In reality, however, the tetramers form a network which is anchored to both the membrane through polypeptides called ankyrin, and to proteins of the cytoskeleton. It is thought that this scaffold-like structure helps to maintain the biconcave shape of erythrocytes while allowing enough flexibility for the cells to squeeze through small capillaries.  Hereditary spherocytosis is a genetic disease that leads to the lack of both ankyrin and spectrin.  In erythrocytes lacking these proteins, portions of the phospholipid bilayer form vesicles and are lost from the RBC surface resulting in decreased surface area and spherocytosis.

822/04 Copyright (C) 2004, Jonathan Monroe, monroejd@jmu.edu. All rights reserved.
URL: http://csm.jmu.edu/biology/courses/bio220/spectrin.html