A Novel Regulator Of G-Protein Signaling Bearing Gap Activity For G Alpha I And G Alpha Q In Megakaryocytes

The regulator of G-protein signaling (FIGS) negatively regulates the alpha subunit of G proteins by accelerating their intrinsic guanosine triphosphatase (GTPase) activity. Here are reported the Isolation and characterization of a novel mouse RGS, termed RGS18, which is a new member of RGS subfamily B. Northern blot analysis showed that RGS18 messenger RNA was detected predominantly in spleen and hematopoietic cells, and immunohistochemical studies demonstrated that RGS18 was expressed in megakaryocytes, platelets, granulocytes/monocytes, and, weakly, in hematopoietic stem cells, but not in lymphocytes or erythrocytes. Although various subcellular localizations of RGS have been reported, RGS18 was found to be localized in cytoplasm in megakaryocytes. In vitro binding assays of RGS18 with megakaryocyte cell lysates with or without AIF(4)(-) treatment demonstrated that RGS18 specifically binds to 2 alpha subunits of the G protein, G alphai and G alphaq. Furthermore, RGS18 clearly exhibited GTPase-activating protein (GAP) activity for G alphai and G alphaq but not for G alphas or G alpha 12. In addition, chemokine stromal-derived factor 1 (SDF-1), which has been reported to stimulate megakaryocyte colony formation in the presence of thrombopoietin, affected the binding of RGS18 to G alphai but not to G alphaq. Therefore, the newly isolated RGS18 turned out to be a new member of the RGS family bearing GAP activity for G alphai, which might be stimulated by SDF-1 in megakaryocytes, as well as for G alphaq. Thus, RGS18 may play an important role in proliferation, differentiation, and for migration of megakaryocytes. (Blood. 2001;97:3051-3060) (C) 2001 by The American Society of Hematology.