MOLECULAR EVOLUTION OF INTEGRINS - GENES ENCODING INTEGRIN BETA-SUBUNITS FROM A CORAL AND A SPONGE

The integrin family of cell surface receptors is strongly conserved in higher animals, but the evolutionary history of integrins is obscure, We have identified and sequenced cDNAs encoding integrin beta subunit s from a coral (phylum Cnidaria) and a sponge (Porifera), indicating t hat these proteins existed in the earliest stages of metazoan evolutio n, The coral beta(Cn1) and, especially, the sponge beta(Po1) sequences are the most divergent of the ''beta(1)-class'' integrins and share a number of features not found in any other vertebrate or invertebrate integrins, Perhaps the greatest difference from other beta subunits is found in the third and fourth repeats of the cysteine-rich stalk, whe re the generally conserved spacings between cysteines are highly varia ble, but not similar, in beta(Cn1) and beta(Po1). Alternatively splice d cDNAs, containing a stop codon about midway through the full-length translated sequence, were isolated from the sponge library, These cDNA s appear to define a boundary between functional domains, as they woul d encode a protein that includes the globular ligand-binding head but would be missing the stalk transmembrane, and cytoplasmic domains. The se and other sequence comparisons with vertebrate integrins are discus sed with respect to models of integrin structure and function.