Identification of multiple integrin β1 homologs in zebrafish (Danio rerio)

Background: Integrins comprise a large family of alpha,beta heterodimeric, transmembrane cell adhesion receptors that mediate diverse essential biological functions. Higher vertebrates possess a single beta 1 gene, and the beta 1 subunit associates with a large number of alpha subunits to form the major class of extracellular matrix (ECM) receptors. Despite the fact that the zebrafish ( Danio rerio) is a rapidly emerging model organism of choice for developmental biology and for models of human disease, little is currently known about beta 1 integrin sequences and functions in this organism. Results: Using RT-PCR, complete coding sequences of zebrafish beta 1 paralogs were obtained from zebrafish embryos or adult tissues. The results show that zebrafish possess two beta 1 paralogs (beta 1-1 and beta 1-2) that have a high degree of identity to other vertebrate beta 1 subunits. In addition, a third, more divergent, beta 1 paralog is present (beta 1-3), which may have altered ligand-binding properties. Zebrafish also have other divergent beta 1-like transcripts, which are C-terminally truncated forms lacking the transmembrane and cytoplasmic domains. Together with beta 1-3 these truncated forms comprise a novel group of beta 1 paralogs, all of which have a mutation in the ADMIDAS cation-binding site. Phylogenetic and genomic analyses indicate that the duplication that gave rise to beta 1-1 and beta 1 2 occurred after the divergence of the tetrapod and fish lineages, while a subsequent duplication of the ancestor of beta 1-2 may have given rise to beta 1-3 and an ancestral truncated paralog. A very recent tandem duplication of the truncated beta 1 paralogs appears to have taken place. The different zebrafish beta 1 paralogs have varied patterns of temporal expression during development. beta 1-1 and beta 1-2 are ubiquitously expressed in adult tissues, whereas the other beta 1 paralogs generally show more restricted patterns of expression. Conclusion: Zebrafish have a large set of integrin beta 1 paralogs. beta 1-1 and beta 1-2 may share the roles of the solitary beta 1 subunit found in other vertebrates, whereas beta 1-3 and the truncated beta 1 paralogs may have acquired novel functions.