HMGB2 protein from the marine sponge Suberites domuncula

Investigation of the phylogenetically conserved genes/proteins from marine sponges (Porifera), the most primitive metazoan phylum, can be applied for the reconstruction of ancient structures of genome/proteom complexity in the ancestral organism common to all multicellular animals. The complete nucleotide sequence of Suberites domuncula (Demospongiae) cDNA coding for 183-amino-acid protein (21.3 kDa), which displays high overall similarity in primary structure and organization of domains with HMGB canonical proteins, belonging to High Mobility Group (HMG) family of nuclear proteins, is reported here. The major role of these non-specific DNA binding proteins is to facilitate the formation of complex nucleoprotein assemblies and nucleosome remodelling. The encoded protein, named HMGB2SD, contains two typical, highly conserved DNA-binding domains: box A (69 aa) and box B (71 aa). Short C-terminal >>tail<< is only 13 aa long. HMG2SD displays the highest overall similarity (58%) with HMGB2 proteins from mammals (pig, human, rat, mouse), higher than with Drosophila melanogaster (53%) or Caenorhabditis elegans (31%) homologues. This is in accordance with previous results, which showed the best homology between sponge and mammalian homologues/ortologues. Our results further confirm that sponges are an excellent model for molecular evolutionary studies.