Natriuretic peptide receptor A activation stabilizes a membrane-distal dimer interface

We have shown previously (Rondeau, J.-J., McNicoll, N., Gagnon, J., Bouchard, N., Ong, H., and De Lean, A. (1995) Biochemistry 34,2130-2136) that atrial natriuretic peptide (ANP) stabilizes a dimeric form of the natriuretic peptide receptor A (NPRA) by simultaneously interacting with both receptor subunits. However, the first crystallographic study of unliganded NPRA extracellular domain documented a V-shaped dimer involving a membrane-proximal dimer interface and separate binding sites for ANP on each monomer. We explored the possibility of an alternative A-shaped dimer involving a membrane-distal dimer interface by substituting an unpaired solvent-exposed cysteine for Trp74 in the amino-terminal lobe of full-length NPRA. The predicted spacing between Trp(74) from both subunits drastically differs, depending on whether the V-shaped (84 Angstrom) or the A-shaped (8 Angstrom) dimer model is considered. In contrast with the expected results for the reported V-shaped dimer, the NPRA(W74C) mutant was constitutively covalently dimeric. Also, the"subunits spontaneously associated following transient disulfide reduction by dithiothreitol and reoxidation. However, ANP could neither bind to nor activate NPRA(W74C). Permanent disulfide opening by reduction with dithiothreitol and alkylation with N-ethylmaleimide rescued ANP binding to NPRA(W74C). The NPRA mutant could be maintained as a covalent dimer while preserving its function by crosslinking with the bifunctional alkylating agent phenylenedimaleimides (PDM), the ortho-substituted oPDM being more efficient than mPDM or pPDM. These results indicate that the membrane-distal lobe of the NPRAM extracellular domains are dynamically interfacing in the unliganded state and that ANP binding stabilizes the receptor dimer with more stringent spacing at the dimer interface.