CLONING, PROPERTIES, SITE-DIRECTED MUTAGENESIS ANALYSIS OF THE SUBUNIT STRUCTURE, TISSUE DISTRIBUTION AND REGULATION OF EXPRESSION OF THE TYPE-C EEL NATRIURETIC PEPTIDE RECEPTOR

Eel natriuretic peptide receptor C (NPR-C) was cloned, characterized a nd found to have a unique interchain disulfide linkage when compared t o that of mammalian NPR-C. The NPR-C cDNA was obtained from an eel gil l cDNA library; the open reading frame codes for a polypeptide of 502 amino acids exhibiting the known features of NPR-C, including a weak l igand specificity and a disulfide-linked homodimeric structure. The de duced amino acid sequence shares approximately 60% similarity with the mammalian NPR-C but it lacks the Gly-rich prosequence present in the mammalian counterparts. Site-directed mutagenesis revealed that eel an d mammalian NPR-C are quite different in their interchain disulfide-bo nding pattern; eel uses the second Cys residue and mammals the fifth C ys residue for the covalent dimerization. The ligand-binding activity of the extracellular domain is not independent of the short cytoplasmi c tail. RNase protection analysis revealed that the eel receptor is hi ghly expressed in the gill and heart and, to a much lesser extent, in other tissues including the brain and intestine. The NPR-C mRNA levels were found to be down-regulated in most tissues when eels were transf erred from fresh water to seawater; however, in the anterior intestine , the levels were up-regulated suggesting that NPR-C plays a role in t he adaptation to salinity changes in the euryhaline eel.