Ras-related small GTPase Rab21 from the marine sponge Geodia cydonium

Background and Purpose: Sponges belong to the most ancient and the most simple extant phylum of multicellular animals - Porifera. We recently analyzed numerous sponge cDNAs and genes encoding evolutionary conserved proteins, and found that sponge proteins are most similar to orthologues/homologues from mammals. Here, we describe the isolation and analysis of a cDNA from the marine sponge Geodia cydonium (Demospongiae), that encodes a protein belonging to the Ras superfamily of small GTPases, Materials and Methods: The cDNA clone was isolated from G. cydonium cDNA library prepared in lambdaZAP Express vector by standard hybridization methods and sequenced on an automatic DNA sequencer. The encoded protein was analyzed in details and compared with the related proteins. Results: Nucleotide sequence of the cDNA predicts a protein of 229 amino acid residues (calculated M, 25344). This sponge protein displays highest structural homology to GTP-binding proteins from the Rab subfamily, that play an important role in intracellular vesicular targeting. The presence of a glycine at position 25in the first GTP binding region ((23)GEGAVGKT(30)) and the sequence (45)HEQTLQASF(53) classify the sponge protein as a Rab21 GTPase. Metazoan Rab21 proteins were so far described only from mammals (human and dog) and from sponge in this study. G. cydonium. Rab21 displays 61% identity (75% homology) with human/dog Rab21 proteins, while putative genes for Rab21, identified in the Caenorhabditis elegans and Drosophila melanogaster genomes, code for proteins with only around 50% identity to mammalian Rab21. Conclusion: G. cydonium. Rab21 GTPase is another example of an evolutionary conserved sponge protein that shows an unusually high degree of similarity with mammalian proteins.