Integrin evolution: Insights from ascidian and teleost fish genomes

Integrins are a family of αβ heterodimeric receptors essential to cell adhesion in all metazoans. In humans, the family consists of 18 α and 8 β subunits that combine to form 24 dimers. Here, we present phylogenetic reconstructions for the α and β 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 α subunits that have the inserted αI domain. As for the one αI domain containing integrin α subunit discovered earlier from the ascidian Halocynthia roretzi, the Ciona αI domains are missing the distinctive characteristics of mammalian collagen receptors and segregate from all vertebrate αI domain integrins in a phylogenetic tree, forming a new subgroup of α subunits with αI domains. Each of the pufferfish αI domain sequences does have characteristics of the collagen receptor αI domains, but no leukocyte-specific αI domains were found in pufferfish. Comparative protein modeling suggests that several of these fish αI domains are structurally compatible with binding to a GFOGER sequence in a collagen triple helix.