RGS (regulator of G protein signaling) proteins are GTPase-activating proteins (GAPs) for heterotrimeric G protein alpha subunits and negatively regulate G protein-mediated signal transduction. In this study, we determined the cDNA sequence of a novel Caenorhabditis elegans (C. elegans) RGS protein. The predicted protein, termed C2-RGS, consists of 782 amino acids, and contains a C2 domain and an RGS domain. C2 domains are typically known to be Ca(2+) and phospholipid binding sites, found in many proteins involved in membrane traffic or signal transduction, and most of their biological roles are not identified. To study the function of C2-RGS protein, a series of six truncated versions of C2-RGS were constructed. When the full-length protein of C2-RGS was expressed transiently in AT1a-293T cells, ET-1-induced Ca(2+) responses were strongly suppressed. When each of the mutants with either RGS domain or C2 domain was expressed, the Ca(2+) responses were suppressed moderately. Furthermore, we found that C2 domain of PLC-beta1 also had a similar moderate inhibitory effect. RGS domain of C2-RGS bound to mammalian and C. elegans Galphai/o and Galphaq subunits only in the presence of GDP/AlF(4)(-), and had GAP activity to Galphai3. On the other hand, C2 domains of C2-RGS and PLC-beta1 also bound strongly to Galphaq subunit, in the presence of GDP, GDP/AlF(4)(-), and GTPgammaS, suggesting the stable persistent association between these C2 domains and Galphaq subunit at any stage during GTPase cycle. These results indicate that both the RGS domain and the C2 domain are responsible for the inhibitory effect of the full-length C2-RGS protein on Galphaq-mediated signaling, and suggest that C2 domains of C2-RGS and PLC-beta1 may act as a scaffold module to organize Galphaq and the respective whole protein molecule in a stable signaling complex, both in the absence and presence of stimulus.