Ankyrins are a family of proteins called repeat proteins that contain ankyrin-like repeats in their structure. Proteins in which ankyrin repeats are present are binding proteins that interact with integral membrane proteins via dynamic non-covalent bonds and help determine their distribution within the cell membrane. They play a key role in activities such as cell motility, activation, proliferation, contact and the maintenance of specialized membrane domains.
Ankyrin
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Most ankyrins are typically composed of three structural domains: an amino-terminal domain containing multiple ankyrin repeats, a central region with a highly conserved spectrin-binding domain, and a carboxy-terminal regulatory domain that is the least conserved and subject to variation.
Thus, multiple isoforms of ankyrin are now known with different affinities for different target proteins. They are expressed in a tissue-specific, developmentally regulated manner. In erythrocytes, for example, ankyrin binds to the cell membrane via spectrin (Michaely P et al. 2002). The sarcoplasmic small ankyrin 1 protein (sANK1) interacts with connectin (titin), thereby positioning the sarcoplasmic reticulum (SR) to the Z-disk of the sarcomere. Titin is the largest protein in mammals, up to 3.7 MDa, and forms the third filament system in cardiac and skeletal muscle, along with actin and myosin. This documents the important physiological role of sANK1.
The pathogen Plasmodium falciparum also binds to ankyrin.
Specific ankyrin antibodies bind to a Treponema pallidum acidic repeat protein and are used to detect Treponema pallidum.
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Mutations in genes encoding ankyrin-like proteins can cause defects in gene expression that lead to the onset and progression of some human and animal diseases. For example, half of the cases of "hereditary spherocytosis" are based on a mutation (deletion) in the ankyrin gene (Agarwal AM (2019). The genetic defect of ankyrin on the inner surface of the erythrocyte membrane leads to decreased membrane stability. The decreased membrane stability leads to decreased osmotic resistance of erythrocytes. This results in increased degradation in the spleen with hemolytic anemia and splenomegaly.
In some forms (about 2%) of dilated cardiomyopathy, mutant proteins with "ankyrin-repeats" in their structure have also been detected. The "ankyrin repeat domain 1 gene (ANKRD1)", was identified as a disease gene in 2009. The gene encodes the cardiac ankyrin repeat protein (CARP) with a molecular weight of approximately 36 kDa. Three different functionally intact mRNA transcripts of ANKRD-1 are known and translated: ankrd1 - ankrd3. However, the functional significance of the variants is currently still unclear.