Beta-III spectrin is a protein encoded by the gene SPTBN2, spans 2390 amino acids and consists of an amino-terminal actin binding domain (ABD), a central region containing seventeen spectrin repeat domains, and a carboxy-terminal pleckstrin homology domain. It is critical for the correct development and maintenance of Purkinje cell dendritic structure. Beta-III spectrin is expressed predominantly in the brain and is enriched in cerebellar Purkinje cells. In addition, Beta-III spectrin participates in endomembrane trafficking through its interaction with the actin related protein, ARP1. The functional unit of Beta-III spectrin is considered to be a tetrameric complex composed of two Beta-spectrin subunits and two Alpha-II-spectrin subunits. Mutations in the gene encoding Beta-III spectrin give rise to spinocerebellar ataxia type 5, a neurodegenerative disease characterized by progressive thinning of the molecular layer, loss of Purkinje cells and increasing motor deficits.
