Integrin evolution: Insights from ascidian and teleost fish genomes

Integrins are a family of alpha beta 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 alpha I domain. As for the one alpha I domain containing integrin alpha subunit discovered earlier from the ascidian Halocynthia roretzi, the Ciona alpha I domains are missing the distinctive characteristics of mammalian collagen receptors and segregate from all vertebrate alpha I domain integrins in a phylogenetic tree, forming a new subgroup of alpha subunits with alpha I domains. Each of the pufferfish alpha I domain sequences does have characteristics of the collagen receptor alpha I domains, but no leukocyte-specific alpha I domains were found in pufferfish. Comparative protein modeling suggests that several of these fish alpha I domains are structurally compatible with binding to a GFOGER sequence in a collagen triple helix. (c) 2005 Elsevier B.V./International Society of Matrix Biology. All rights reserved.