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Blood, 15 May 2001, Vol. 97, No. 10, pp. 3051-3060
HEMATOPOIESIS
A novel regulator of G-protein signaling bearing GAP activity
for G i and Gq in megakaryocytes
Yuka Nagata,
Masaaki Oda,
Hiroko Nakata,
Yuka Shozaki,
Tohru Kozasa, and
Kazuo Todokoro
From the Tsukuba Life Science Center, The Institute of
Physical and Chemical Research, Japan, and the Department of
Pharmacology, University of Illinois at Chicago.
The regulator of G-protein signaling (RGS) negatively regulates the
 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
AlF4 treatment demonstrated that
RGS18 specifically binds to 2 subunits of the G protein, Gi and
Gq. Furthermore, RGS18 clearly exhibited GTPase-activating protein
(GAP) activity for Gi and Gq but not for Gs or G12. 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 Gi but not to
Gq. Therefore, the newly isolated RGS18 turned out to be a new
member of the RGS family bearing GAP activity for Gi, which might be
stimulated by SDF-1 in megakaryocytes, as well as for Gq. Thus,
RGS18 may play an important role in proliferation, differentiation,
and/or migration of megakaryocytes.
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