[
Curr Biol,
1999]
Guanylyl cyclases (GCs), the enzymes that synthesize cyclic GMP (cGMP), have classically been divided into two groups: receptor GCs and soluble GCs. Receptor GCs are integral membrane proteins such as the retinal guanylyl cyclases and the atrial natriuretic peptide receptors. Soluble GCs are heterodimeric proteins consisting of an alpha and a beta subunit that bind a heme group and are activated by nitric oxide (NO). The recent publication of the Caenorhabditis elegans genomic sequence has revealed an unexpected abundance of GCs.