Alpha-actinin - an amazing journey through time and species

Abstract: In eukaryotes, the actin cytoskeleton plays an important role in a large variety of cellular events. Its reorganization is regulated by a plethora of actin-modulating proteins, such as ?-actinin.?-actinin is a ubiquitous actin-binding protein that belongs to the spectrin superfamily. This family, besides ?-actinin, includes spectrin, dystrophin and utrophin. Phylogenetic analyses have indicated that the family members arose after several intragene duplications and rearrangements of a common ancestral ?-actinin isoform. Up to the invertebrate-vertebrate bifurcation, organisms seemed to have a single, calcium-dependent ??actinin. After the split, invertebrates have kept this single isoform, in contrast to vertebrates that acquired four distinct isoforms. Of the four vertebrate ?-actinin isoforms, the two present in non-muscle cells are typically calcium sensitive while the two muscle isoforms are calcium insensitive.?-actinin in higher organisms is characterized by the presence of three distinct structural domains: a highly conserved N-terminal actin-binding domain, a central rod domain with four spectrin repeats and a calcium-binding C terminus with EF-hand motifs. In some primitive organisms, such as protozoa and fungi, the rod domain of ?-actinin contains only one or two spectrin repeats. With the completion of an ever increasing number of genomes, new and atypical ??actinin sequences had been available that have not been characterized yet. To obtain a firmer understanding of the evolutionary history of ?-actinin, the main objective of this study was to identify, purify and biochemically characterize atypical ??actinin or ??actinin-like proteins of the parasite Entamoeba histolytica and of the fungus Schizosaccharomyces pombe. Our results show that both isoforms, despite the much shorter rod domain, are able to bind and cross-link actin filaments and therefore can be considered genuine ?-actinins.