PHYLOGENETIC RELATIONSHIP OF ADHESION PROTEINS AND UBIQUITIN FROM THE MARINE SPONGE GEODIA-CYDONIUM

The shift from unicellular life to multicellular, integrated organisms has been accompanied by the acquisition of adhesion proteins. 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 lectins; cDNA for two of them (termed LECT-1 and LECT-2) was cloned. Both lectins have a framework sequence of 38 conserved amino acids which are characteristic for the carbohydrate binding site of vertebrate S-type lectins. Next, we have isolated and characterized a cDNA coding for a receptor tyrosine kinase of class II (GCTK). The deduced aa sequence shows two characteristic domains; (i) the tyrosine kinase domain and (ii) an immunoglobulin-like domain. The latter part shows high homology to the vertebrate type immunoglobulin domain. This result together with the lectin data demonstrates that binding domains of such adhesion proteins are not recent achievements of higher animals but exist already in animals (sponges) which have diverged from other organisms about 800 million years ago. Ubiquitin is a 76-residue protein which is highly conserved among eukaryotes. Sponge cDNA for poly-ubiquitin was cloned; the sponge gene (GCUBI) contains six repeats, GCUBI-1 to GCUBI-6. On the nt level the sequences of the six repeats differ considerably. Mutational analysis suggests that the sponge polyubiquitin gene evolved from an ancestral monoubiquitin gene by gene duplication and successive tandem duplications. The first event, duplication of the monoubiquitin gene, happened some 110 millions years ago. These data support the view that the kingdom Animalia is monophyletic.