alpha-actinin is a ubiquitous protein found in most eukaryotic organisms. The ability to form dimers allows alpha-actinin to cross-link actin in different structures. In muscle cells alpha-actinin is found at the Z-disk of sarcomeres. In non-muscle cells alpha-actinin is found in zonula adherens or focal adhesion sites where it can bind actin to the plasma membrane.
alpha-actinin is the shortest member of the spectrin superfamily of proteins which also includes spectrin, dystrophin and utrophin. Several hypotheses suggest that alpha-actinin is the ancestor of this superfamily.
The structure of alpha-actinin in higher organisms has been well characterized consisting of three main domains: an N-terminal actin-binding domain with two calponin homology domains, a central rod domain with four spectrin repeats and a C-terminal calcium-binding domain. Data mining of genomes from diverse organisms has made possible the discovery of new and atypical alpha-actinin isoforms that have not been characterized yet.
Invertebrates contain a single alpha-actinin isoform, whereas most of the vertebrates contain four. These four isoforms can be broadly classified in two groups, muscle isoforms and non-muscle isoforms. Muscle isoforms bind actin in a calcium independent manner whereas non-muscle isoforms bind actin in a calcium-dependent manner.
Some of the protozoa and fungi isoforms are atypical in that they contain fewer spectrin repeats in the rod domain. We have purified and characterized two ancestral alpha-actinins from the parasite Entamoeba histolytica. Our results show that despite the shorter rod domain they conserve the most important functions of modern alpha-actinin such as actin-bundling formation and calcium-binding regulation. Therefore it is suggested that they are genuine alpha-actinins.
The phylogenetic tree of alpha-actinin shows that the four different alpha-actinin isoforms appeared after the vertebrate-invertebrate split as a result of two rounds of genome duplication. The atypical alpha-actinin isoforms are placed as the most divergent isoforms suggesting that they are ancestral isoforms. We also propose that the most ancestral alpha-actinin contained a single repeat in its rod domain. After a first intragene duplication alpha-actinin with two spectrin repeats were created and a second intragene duplication gave rise to modern alpha-actinins with four spectrin repeats.