In mammals, cyclic GMP and cGMP-dependent protein kinases (cGKs) have been implicated in the regulation of many neuronal functions including long-term potentiation and long-term depression of synaptic efficacy. To develop Caenorhabditis elegans as a model system for studying the neuronal function of the cGKs, we cloned and characterized the cgk-1 gene. A combination of approaches showed that cgk-1 produces three transcripts, which differ in their first exon but are similar in length. Northern analysis of C. elegans RNA, performed with a probe designed to hybridize to all three transcripts, confirmed that a major 3.0 kb cgk-1 transcript is present at all stages of development. To determine if the CGK-1C protein was a cGMP-dependent protein kinase, CGK-1C was expressed in SF:9 cells and purified. CGK-1C shows a K(a) of 190 +/- 14 nM for cGMP and 18.4 +/- 2 microM for cAMP. Furthermore, CGK-1C undergoes autophosphorylation in a cGMP-dependent manner and is inhibited by the commonly used cGK inhibitor, KT5823. To determine which cells expressed CGK-1C, a 2.4-kb DNA fragment from the promoter of CGK-1C was used to drive GFP expression. The CGK-1C reporter construct is strongly expressed in the ventral nerve cord and in several other neurons as well as the marginal cells of the pharynx and intestine. Finally, RNA-mediated interference of CGK-1 resulted in movement defects in nematode larvae. These results provide the first demonstration that cGMP-dependent protein kinase is present in neurons of C. elegans and show that this kinase is required for normal motility.