The spectrin family of proteins were originally discovered as major components of the submembraneous cytoskeleton of osmotically lyzed red blood cells (1). The lyzed blood cells could be seen as clear red blood cell shaped objects in the light microscope and were referred to as red cell "ghosts". The major proteins of these ghosts proved to be actin, ankyrin, band 4.1 and several other proteins, including two major bands running at about 240kD and 260kD on SDS-PAGE gels. This pair of bands was named "spectrin" since they were discovered in these red blood cell ghosts (1). Later work showed that similar high molecular bands were seen in membrane preparations from other eukaryotic cell types. Work by Levine and Willard described a pair of about ~240-260kD molecular weight bands which were transported at the slowest rate along mammalian axons (2). They named these proteins "fodrin" as antibody studies showed that they were localized in the sheath under the axonal membrane, but not in the core of the axon (2; fodros is Greek for sheath). Subsequently fodrin was found to be a member of the spectrin family of proteins, and the spectrin nomenclature is now normally used (3). Spectrins form tetramers of two alpha and two beta subunits, with the alpha corresponding to the lower molecular weight ~240kD band and the beta corresponding to the ~260kD or in some case much larger band. Most spectrin tetramers are about 0.2microns or 200nm long, and each alpha and beta subunit has a cell type specific expression pattern. The basic structure of each spectrin subunit is the spectrin repeat, which is a sequence of about 110 amino acids which defines a compact domain contain three closely packed alpha-helices. Each spectrin subunit contains multiple copies of this repeat, with 20 in each of the alpha subunits. The beta I-IV subunits each contain 17 spectrin repeats, while the beta V subunit, also known as beta-heavy spectrin, contains 30 of these repeats. The various subunits also contain several other kinds of functional domain, allowing the spectrin tetramer to interact with a variety of protein, ionic and lipid targets. The alpha-subunits each contain one calmodulin like calcium binding region and one Src-homology 3 (SH3) domain, an abundant domain involved in specific protein-protein interactions. The beta subunits all have a N-terminal actin binding domain and may also have one SH3 domain and one pleckstrin homology domain, a multifunctional type of binding domain which in beta I spectrin at least binds the membrane lipid PIP2 (5). Spectrins are believed to have a function in giving mechanical strength to the plasma membrane since the tetramers associate with each other to form a dense submembraneous geodesic meshwork (3). They also bind a variety of other membrane proteins and membrane lipids, and the proteins they bind to are therefore themselves localized in the membrane. Diseases may be associated with defects in one or other of the spectrin subunits (6). For example, some forms of hereditary spherocytosis, the presence of spherical red blood cells which are prone to lysis, can be traced to mutations in some of the spectrin subunits (7). The alpha-II subunit is widely expressed in tissues but, in the nervous system, is found predominantly in neurons. The HGNC name for this protein is SPTAN1.
Intended for research use only. Not for use in human, therapeutic, or diagnostic applications.