Integrin evolution: insights from ascidian and teleost fish genomes.

Integrins are a family of alphabeta heterodimeric receptors essential to cell adhesion in all metazoans. In humans, the family consists of 18 alpha and 8 beta subunits that combine to form 24 dimers. Here, we present phylogenetic reconstructions for the alpha and beta integrin subunits based on sequences from 24 invertebrate and vertebrate species, including the fully sequenced genomes of the ascidian Ciona intestinalis (a urochordate) and the pufferfish Takifugu rubripes (a teleost). Both genomes contain integrin alpha subunits that have the inserted alphaI domain. As for the one alphaI domain containing integrin alpha subunit discovered earlier from the ascidian Halocynthia roretzi, the Ciona alphaI domains are missing the distinctive characteristics of mammalian collagen receptors and segregate from all vertebrate alphaI domain integrins in a phylogenetic tree, forming a new subgroup of alpha subunits with alphaI domains. Each of the pufferfish alphaI domain sequences does have characteristics of the collagen receptor alphaI domains, but no leukocyte-specific alphaI domains were found in pufferfish. Comparative protein modeling suggests that several of these fish alphaI domains are structurally compatible with binding to a GFOGER sequence in a collagen triple helix.