CELL-ADHESION RECEPTORS AND NUCLEAR RECEPTORS ARE HIGHLY CONSERVED FROM THE LOWEST METAZOA (MARINE SPONGES) TO VERTEBRATES

The shift from unicellular life to multicellular, integrated organisms has been accompanied by the acquisition of adhesion proteins/receptor s. Recently we succeeded to clone some genes coding for such proteins from the lowest multicellular animals, the marine sponges (model: the siliceous sponge Geodia cydonium). G. cydonium contains e.g. several l ectins; cDNAs for two of them were cloned. Both lectins have a framewo rk sequence of 38 conserved amino acids which are characteristic for t he carbohydrate binding site of vertebrate S-type lectins. Next, the c DNA coding for a receptor tyrosine kinase of class II was isolated and characterized. The deduced aa sequence shows two characteristic domai ns; (i) the tyrosine kinase domain and (ii) an immunoglobulin-like dom ain. The latter part displays high homology to the vertebrate type imm unoglobulin domain. This result together with the lectin data demonstr ates that binding domains of such adhesion proteins are not recent ach ievements of higher animals but exist already in animals (sponges) whi ch have diverged from other organisms about 800 million years ago. Con sidering the fact that during embryogenesis of sponges a typical anter oposterior organization pattern is seen a 'homeotic' organ-like transf ormation has been postulated. The subsequent search for genes provided with the homeodomain-like sequence was successful. These data support the view that the kingdom Animalia is of monophyletic origin.