<p>Guanylate cyclases (<db_xref db="EC" dbkey="4.6.1.2"/>) catalyse the formation of cyclic GMP (cGMP) from GTP. cGMP acts as an intracellular messenger, activating cGMP-dependent kinases and regulating cGMP-sensitive ion channels. The role of cGMP as a second messenger in vascular smooth muscle relaxation and retinal photo-transduction is well established. Guanylate cyclase is found both in thesoluble and particulate fractions of eukaryotic cells. The soluble and plasmamembrane-bound forms differ in structure, regulation and other properties [<cite idref="PUB00000129"/>,<cite idref="PUB00000877"/>, <cite idref="PUB00001511"/>, <cite idref="PUB00004322"/>]. Most currently known plasma membrane-boundforms are receptors for small polypeptides. The soluble forms of guanylate cyclase arecytoplasmic heterodimers having alpha and beta subunits.</p><p> In all characterised eukaryote guanylyl- and adenylyl cyclases, cyclic nucleotide synthesis is carried out by the conserved class III cyclase domain. </p>