Function 
Spectrin forms scaffolding in plasma membranes and cytoskeletal structure.  It interacts with actin at either end of its tetramer[1].  The SPT dimer is formed by association of α1 and β1 monomers.  In invertebrates there are SPT α, β and βH.  In vertebrates there are SPT α1 (SPTA1), α2 (SPTA2) and β1 (SPTB1) to β5.  SPT contains an SRC Homology 3 domain (SH3), a Pleckstrin Homology (PH) domain and a Calponin Homology (CH) domain. 
  Disease 
Mutations in SPT α are found in patients with hereditary elliptocytosis[2]. SPT β deficiency is found in hereditary spherocytosis[3]. 
  3D Structures of Spectrin 
Spectrin 3D structures