Ankyrins

Ankyrins are a family of conserved proteins whose prime function is to act as a link between integral membrane proteins and the spectrin- or fodrin-based framework lying on the cytoplasmic side of the plasmalemma. The ankyrins were first characterized as those proteins that displayed a strong affinity for spectrin in erythrocyte membranes. They are expressed in particularly high levels in vertebrate brain but are also found in tissues such as skeletal muscle, lymphocytes, neutrophils and many epithelial tissues (1); they are also found in such primitive organisms as worms (nematodes) and fruit flies (Drosophila). Associations between ankyrins and a diverse selection of membrane proteins such as cell adhesion molecules and ion channels (the anion exchanger, or band 3 protein, Na+/K+ ATPase and the voltage-sensitive sodium channel) have been characterized and confirm that a key role of the ankyrins is to mediate interactions between spectrin in erythrocytes (known as fodrin in other cells) and proteins constituting the cell membrane. Deficiencies in ankyrin (as well as spectrin and some other membrane-associated proteins) have been correlated with structurally weak erythrocytes.

Ankyrins from erythrocytes (Ank1) and brain (Ank2) are monomeric proteins with a simple domain substructure. Each protein has an N-terminal domain (about 89 to 95 kDa) with membrane-binding abilities that can interact with the anion exchanger and tubulin (amongst others). Most of this globular region comprises 24 tandem quasi-repeats of a 33-residue motif. Because six such repeats appear to represent the smallest structural entity, it is possible that there are four subdomains within the N-terminal domain. It has also been suggested that the 33-residue structures, while folding up in a common manner, could position nonconserved residues on surface sites that would enable them to interact with a wide range of different protein ligands, rather than with a single ligand multiple times. The sequence of the 33-residue motif is very similar to those found in transcription factors, in cell-cycle control proteins, and in proteins regulating tissue differentiation. The second domain of ankyrin (about 62 kDa) can be subdivided into two parts; the first is acidic, proline-rich, and about 80 residues in length. In erythrocytes it links the cytoplasmic domain of the transmembrane anion exchanger to b-spectrin at about the midpoint of the tetrameric structure that b-spectrin forms with a-spectrin (see Spectrin). The second portion is basic and much larger (almost 500 residues). It contains the consensus sequence (Arg–Arg–Arg–Lys–Phe–His–Lys/Arg) that is also required for spectrin- or fodrin-binding. It is also much more highly conserved in sequence than the acidic 80-residue subdomain. The C-terminal domain is about 70 kDa in both Ank1 and one variant of Ank2, but about 290 kDa in an isoform of Ank2 that is generated by alternative splicing. The smaller isoform of Ank2 (molecular weight about 220 kDa) is common in adult rats, while the larger one ) about 440 kDa) is expressed highly in neonatal animals. Furthermore, the 220-kDa protein is found widely in the brain (neuron cell bodies, dendrites and glia), whereas the 440-kDa protein seems to be found specifically in unmyelinated axons and dendrites (2). In both Ank1 and Ank2, the C-terminal domain has a regulatory role arising from its ability to modulate the binding affinities of both the membrane-binding and spectrin-binding domains. The sequences of the C-terminal domains differ in size, and the overall homology is not high, although there are some regions of similarity.

Ank3, which is the major ankyrin in kidney, is found in most epithelial cells, axons and muscle cells and has many structural similarities to Ank1 and Ank2. It too has an N-terminal domain (89 kDa(containing a 33-residue repeat with the consensus sequence (Asp/Asn–Gly–apolar–Thr–Pro/Ala–Leu–His–apolar–Ala–Ala–X–X–Gly–His/Asn–apolar–X–Val/Ile–Val/Ala–X–apolar–Leu–Leu–X–X–Gly–Ala–X–Apolar/Pro–Asn/Asp–Ala–X–Thr–Basic), a 65-kDa domain with spectrin-binding ability, and a C-terminal domain (56 kDa) with regulatory attributes (2). Interestingly, however, multiple transcripts are expressed, some of which lack the repeat domain at the N-terminal end of the molecule. It is possible that these ankyrins are involved in intracellular vesicle stabilization, sorting, or targeting (2).

References

1. V. Bennett and D. M. Gilligan (1993) The spectrin-based membrane skeleton and micron-scale organization of the plasma membrane. Annu. Rev. Cell Biol. 9, 27–66. 

2. L. L. Peters, K. M. John, F. M. Lu, E. M. Eicher, A. Higgins, M. Yialamas, L. C. Turtzo, A. J. tsuka, and S. E. Lux (1995) Ank3 (epithelial ankyrin), a widely distributed new member of the ankyrin gene family and the major ankyrin in kidney, is expressed in alternatively spliced forms, including forms that lack the repeat domain. J. Cell Biol. 130, 313–330.