MOLECULAR-CLONING AND PRIMARY STRUCTURE OF A RHESUS (RH)-LIKE PROTEINFROM THE MARINE SPONGE GEODIA-CYDONIUM

In humans, the 30 000 M-r Rhesus (Rh) polypeptide D (RhD) is a dominan t antigen (Ag) of the Rh blood group system. To date, an Rh-like prote in has been found in chimpanzees, gorillas, gibbons, End rhesus monkey s. Related to the 30 000 M-r Rh Ag protein are two polypeptides of 50 000 M-r, the human 50 000 M-r Rh Ag and the RhD-like protein from Caen orhabditis elegans. The function of all these proteins is not sufficie ntly known. Here we characterize a cDNA clone (GCRH) encoding a putati ve 57 000 M-r polypeptide from the marine sponge Geodia cydonium, whic h shares sequence similarity both to the RhD Ag and the Rh50 glycoprot ein. The sponge Rh-like protein comprises 523 aa residues; hydropathy analysis hints at the presence of ten transmembrane domains. An N-term inal hydrophobic cleavage signal sequence is missing, suggesting that the first membrane-spanning domain of the sponge Rh-like protein acts as a signal-anchor sequence. The sponge Rh-like protein, like the huma n Rh50, lacks the CLP motif which is characteristic of the 30 000 M-r RhD. In addition, the hydropathy profile of the sponge Rh-like protein is of a similar size and shape as that of human Rh50. This data indic ates that the RhD and its structurally related Rh50 glycoprotein, whic h are highly immunogenic in humans, share a common ancestral molecule with the G. cydonium Rh-like protein.