A novel regulator of G-protein signaling (RGS) has been isolated from a highly purified population of mouse long-term hematopoietic stem cells, and designated RGS18. It has 234 amino acids consisting of a central RGS box and short divergent NH 2 and COOH termini. The calculated molecular weight of RGS18 is 27,610 and the isoelectric point is 8.63. Mouse RGS18 is expressed from a single gene and shows tissue specific distribution. It is most highly expressed in bone marrow followed by fetal liver, spleen, and then lung. In bone marrow, RGS18 level is highest in long-term and shortterm hematopoietic stem cells, and is decreased as they differentiate into more committed multiple progenitors. The human RGS18 ortholog has a tissue-specific expression pattern similar to that of mouse RGS18. Purified RGS18 interacts with the ␣ subunit of both G i and G q subfamilies. The results of in vitro GTPase single-turnover assays using G␣ i indicated that RGS18 accelerates the intrinsic GTPase activity of G␣ i . Transient overexpression of RGS18 attenuated inositol phosphates production via angiotensin receptor and transcriptional activation through cAMP-responsive element via M1 muscarinic receptor. This suggests RGS18 can act on G q -mediated signaling pathways in vivo.A large number of extracellular stimuli act via cell surface receptors coupled to G-proteins (1). Inactive G-proteins are heterotrimeric proteins consisting of ␣, , and ␥ subunits. Upon binding of a specific ligand to the G-protein-coupled receptor, the receptor promotes the exchange of GDP to GTP in the ␣ subunit, resulting in dissociation of the ␣ subunit from the ␥ subunits. The ␥ subunits are tightly associated and do not dissociate under physiological conditions. The free ␣ subunit and ␥ subunits then transmit signals through various signal transduction pathways. The activated ␣ subunit has slow intrinsic GTPase activity. When the ␣ subunit is in the GDPbound form, it re-associates with the ␥ subunits, leading to an inactive form. The duration of the G-protein signal depends on the rate of GTP hydrolysis and the rate of subunit re-association. For small GTP-binding proteins such as ras, there are GAP 1 proteins (GTPase activating protein), which increase the GTP hydrolysis rate. Recently, functional homologs of the ras-GAP have been identified for the heterotrimeric G-protein.These are called RGS (regulator of G-protein signaling) proteins. The first RGS identified, Sst2 (supersensitivity to pheromone) in yeast, is a negative regulator of pheromone signaling (2). Later, the SST2 gene product was shown to function as a GAP for Gpa1, a molecule involved in pheromone desensitization (3). So far ϳ20 RGS have been identified (4 -12), and more could be anticipated. All RGS proteins have a highly conserved domain consisting of 120 amino acid residues, the RGS box, with varying lengths of NH 2 and COOH termini. RGS4 can be expressed in bacteria, and it has been co-crystallized with G␣ i1 as the GDP-AlF4 Ϫ -bound form (13). It was shown that RGS binds to G␣ i t...