<p>Guanosine cyclase 3',5'-dependent protein kinases are known to play a role in smooth muscle relaxation, ion fluxes in kidneys and intestines, and neuronal function. The predominant form of cGMP-dependent protein kinase is a dimer of identical 75 kDa subunits, although larger subunits of 86 and 130 kDa have been found [<cite idref="PUB00002494"/>]. The enzyme is kept in an inactive form by the interaction of the catalytic domain of one subunit with the region on the other subunit that precedes the cGMP binding domain. Each subunit contains two cGMP-binding regions, found together in the sequence: binding of two molecules of cGMP precipitates a conformational change in the active site that allows the substrate to bind.</p><p>Although cGMP- and cAMP-dependent protein kinases are similar both in structure and sequence around the nucleotide binding site, and in the method of activation and inactivation, there are some basic contrasts. The major difference is that all of the functional domains of the cGMP- dependent enzymes are found on a single polypeptide chain, whereas cAMP- dependent protein kinases have separate regulatory (cAMP binding) and catalytic chains.</p>