Mouse Monoclonal Antibody to αII-Spectrin
Cat# MCA-3D7

      The spectrin family of proteins was originally discovered as the major components of the
submembranous cytoskeleton of osmotically lysed red blood cells (1). The lysed blood cells could be seen as transparent 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 appearing as 240kDa and 260kDa bands on SDS-PAGE gels. This pair of proteins was named “spectrin” since they were discovered in the red blood cell “ghosts” (1). Later work showed that similar high molecular weight bands were seen in membrane fractions from other eukaryotic cell types.

      Work by Levine and Willard described a pair of ~240-260 kDa molecular weight proteins 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 (fodros means sheath in Greek). 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 α and two β subunits, with the α corresponding to the lower molecular weight ~240 kDa band and the β corresponding to the ~260 kDa or in some case much larger band. Most spectrin tetramers are about 0.2 microns or 200 nm long, and each α and β 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 containing three closely packed α-helices. Each spectrin subunit contains multiple copies of this repeat, with 20 in each of the α subunits. The β I-IV subunits each contain 17 spectrin repeats, while the β V subunit, also known as β-heavy spectrin, contains 30 of these repeats. The various subunits also contain several other kinds of functional domains, allowing the spectrin tetramer to interact with a variety of protein, ionic, and lipid targets. The α-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 β 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 β-I spectrin at least binds the membrane lipid PIP2.

      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 submembranous 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 (5). 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 (6). The αII subunit is widely expressed in tissues but, in the nervous system, is found predominantly in neurons. Since the immunogen used to generate the antibody is at the C-terminus of the molecule, the antibody will reveal the 150kDa, 145kDa and 120kDa breakdown products generated by calpain and caspase cleavage on western blots (7). This antibody can also be used to identify neurons and fragments derived from neuronal membranes in cells, in tissue culture, and in sectioned material.