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 and found to have a unique interchain disulfide linkage when compared to that of mammalian NPR‐C. The NPR‐C cDNA was obtained from an eel gill cDNA library; the open reading frame codes for a polypeptide of 502 amino acids exhibiting the known features of NPR‐C, including a weak ligand specificity and a disulfide‐linked homo‐dimeric structure. The deduced 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 and mammalian NPR‐C are quite different in their interchain disulfide‐bonding pattern; eel uses the second Cys residue and mammals the fifth Cys residue for the covalent dimerization. The ligand‐binding activity of the extracellular domain is not independent of the short cytoplasmic tail. RNase protection analysis revealed that the eel receptor is highly 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 transferred from fresh water to seawater; however, in the anterior intestine, the levels were up‐regulated, suggesting that NPR‐C plays a role in the adaptation to salinity changes in the euryhaline eel.