The Cloning and Expression of a New Guanylyl Cyclase Orphan Receptor *

A novel membrane form of guanylyl cyclase (GC-G) has been identified through the isolation of a full-length cDNA clone; it is predicted to contain an extracellular ligand binding domain, a single transmembrane segment, and intracellular protein kinase-like and cyclase catalytic domains. That GC-G represents a guanylyl cyclase was confirmed by both transient expression in COS-7 cells and stable expression in H293 cells. Endogenous cyclic GMP concentrations of transfected or stable cells, however, were much higher than control cells, suggesting an inability of the cells to effectively regulate GC-G cyclase activity. Of six Cys residues found within the extracellular domain of guanylyl cyclase-A (GC-A), the receptor for atrial natriuretic peptide, five are conserved within GC-G. Ligands for the other cyclase receptors, nevertheless, failed to stimulate GC-G expressed in transient or stable cells, suggesting that the unknown ligands possess a structure different from the natriuretic peptides or heat-stable enterotoxins. 125 I-ANP also failed to bind to H293 cells overexpressing GC-G. Based on Northern hybridization, mRNA for GC-G was predominantly expressed in lung, intestine, and skeletal muscle. Using the candidate gene approach to potentially define function, the gene for GC-G was mapped to the distal region of mouse chromosome 19 (syntenic with human chromosome 10q), but no human genetic defect has been ascribed to the GC-G locus. The finding of a new membrane form of guanylyl cyclase in peripheral tissues suggests the existence of another family or subfamily of ligands that signal through elevations of cGMP.