Structural analysis of natriuretic peptide receptor-C by truncation and site-directed mutagenesis

Natriuretic peptide receptor-C (NPR-C) has a unique structure consisting of pre-existing covalent homodimers, but it is not known whether each subunit has ligand-binding activity or whether the dimeric structure is necessary for binding activity. To answer this question, a number of C-terminally truncated mutants were designed, subcloned into the mammalian expression vector pcDNA3 and expressed by transient transfection in COS-1 cells. Truncation at position 461, which eliminates the residue Cys469 that is involved in disulphide-linked dimerization, produced a soluble and monomeric form of NPR-C, as determined by gel filtration on Superose 12. Binding assays of the gel-filtration fractions clearly demonstrated that even monomeric NPR-C contains a high-affinity binding site for natriuretic peptides. Site-directed mutagenesis of the invariant residues (Asp407-Arg408 and Asp411-Phe412) in a region highly conserved among various species established that these invariant residues are essential for ligand-binding activity.