Ankyrins, multifunctional proteins involved in many cellular pathways

Ankyrins, originally discovered as components of the erythrocyte membrane appeared to be a family of animal proteins encoded in mammalian cells by three related genes. Developmentally regulated, tissue specific posttranscriptional processing generates a great variety of isoforms which seem to play specific role in various cells and subcellular structures, being involved, for example, in membrane skeleton organisation, ionic transport, maintenance of cell polarity as well as cell-cell adhesion regulation. The interaction between the membrane skeleton and cytoplasmic domains of transmembrane proteins plays a fundamental role in membrane integrity and stability as well as in many cellular processes. Once the cDNA sequence of red blood cell ankyrin was determined it became clear that "ankyrin-repeat" motifs are present in many proteins whose function is rather unrelated to the membrane skeleton, e.g. transcription factors. Ankyrins are a multigene family of intracellular, structural proteins that link several integral membrane proteins and the spectrin-based membrane cytoskeleton. The anion exchanger, Na+-K+ ATPase, a voltage dependent Na+ channel, an Na+/Ca2+-exchanger, and adhesion molecules have been reported to interact with ankyrin in nonerythroid cells. Ankyrin was first found to link integral membrane proteins to the underlying spectrin-actin based membrane skeleton in the human erythrocyte. It was subsequently described in a variety of vertebrate cells and tissues, including brain, epithelia, and skeletal muscle. Variable cellular localisation of these membrane proteins may be possible due to different relative affinities of various isoforms of ankyrin for target proteins.