Vertebrates have conserved capping protein α isoforms with specific expression patterns

Capping protein (CP), a ubiquitous actin binding protein composed of an α and a β subunit, is important for actin assembly and cell motility. Lower organisms have one gene and one isoform of each subunit. Chickens have two very similar α-subunit isoforms. To determine if vertebrates in general contain multiple α isoforms and if those α isoforms have conserved sequences, we isolated and analyzed α subunit cDNA's in mice and humans. Both mice and humans also have two α isoforms. Phylogenetic analysis of the α isoform sequences reveals that vertebrates have two highly conserved subfamilies, α1 and α2. The α1 and α2 subfamilies are very similar to each other but can be defined and distinguished from each other by a small number of key amino acid residues. In addition, 3' untranslated cDNA sequences are conserved within the isoform subfamilies. To investigate the function of the α isoforms, we examined their expression in mouse cells and tissues. Endothelial cells contain only the α2 isoform, and erythrocytes contain almost exclusively the α1 isoform. Most tissues have both α1 and α2 isoforms but the ratio of α1 : α2 varies widely. Together, these findings support the hypothesis that the CP α isoforms have conserved, unique and essential roles in vertebrates.