Genetics
Principal interactions of structural proteins are at cadherin-based adherens junctions. Actin filaments are linked to α-actinin and to the membrane through vinculin. The head domain of vinculin associates to E-cadherin via α-, β-, and γ-catenins. The tail domain of vinculin binds to membrane lipids and to actin filaments.
The protein actin is one of the most highly conserved throughout evolution because it interacts with a large number of other proteins, with 80.2% sequence conservation at the gene level between Homo sapiens and Saccharomyces cerevisiae (a species of yeast), and 95% conservation of the primary structure of the protein product.
Although most yeasts have only a single actin gene, higher eukaryotes, in general, express several isoforms of actin encoded by a family of related genes. Mammals have at least six actin isoforms coded by separate genes, which are divided into three classes (alpha, beta and gamma) according to their isoelectric points. In general, alpha actins are found in muscle (α-skeletal, α-aortic smooth, α-cardiac, and γ2-enteric smooth), whereas beta and gamma isoforms are prominent in nonmuscle cells (β- and γ1-cytoplasmic). Although the amino acid sequences and in vitro properties of the isoforms are highly similar, these isoforms cannot completely substitute for one another in vivo.
The typical actin gene has an approximately 100-nucleotide 5' UTR, a 1200-nucleotide translated region, and a 200-nucleotide 3' UTR. The majority of actin genes are interrupted by introns, with up to six introns in any of 19 well-characterised locations. The high conservation of the family makes actin the favoured model for studies comparing the introns-early and introns-late models of intron evolution.
All nonspherical prokaryotes appear to possess genes such as MreB, which encode homologues of actin; these genes are required for the cell's shape to be maintained. The plasmid-derived gene ParM encodes an actin-like protein whose polymerised form is dynamically unstable, and appears to partition the plasmid DNA into the daughter cells during cell division by a mechanism analogous to that employed by microtubules in eukaryotic mitosis. Actin is found in both smooth and rough endoplasmic reticulums.
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