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Identification of Gz alpha as a pertussis toxin-insensitive G protein in
human platelets and megakaryocytes
AW Gagnon, DR Manning, L Catani, A Gewirtz, M Poncz and LF Brass
Department of Medicine, University of Pennsylvania, Philadelphia 19104.
G proteins mediate the interaction between cell surface receptors and
intracellular effectors. Recent studies have shown that human retina and
rat brain contain mRNA encoding a novel 40-Kd G protein alpha subunit
referred to as Gz alpha. Studies with an antiserum selective for the
predicted sequence of this protein have suggested that a similar protein is
present in human platelets and is phosphorylated during platelet
activation. To better understand the structure and function of this
protein, the present studies examine its sequence in platelets and compare
its abundance in human platelets, megakaryocytes, and two megakaryoblastic
cell lines, HEL cells and Dami cells. Three different Gz alpha-selective
antisera reacted with a 40-Kd protein in platelet membranes. None of these
detected a corresponding protein in HEL or Dami cells, despite the presence
in both cell lines of proteins recognized by antisera selective for three
members of the Gi alpha family. Northern blotting with a Gz alpha-specific
probe prepared from retinal Gz alpha showed two hybridizing species in
platelet RNA: a major band at 3.5 kb and a minor band at 2.2 kb. Both were
detectable in HEL and Dami cells, but at greatly reduced levels compared
with platelets. RNA encoding Gz alpha was also detected in individual human
megakaryocytes by in situ hybridization. The amount present approached that
of Gi alpha 2' the most abundant of the Gi alpha species present in
platelets. The complete sequence of the platelet homolog to Gz alpha was
determined from platelet RNA amplified by the polymerase chain reaction.
The encoded protein was the same as those obtained in brain and retina.
Thus, based on immunoreactivity and nucleotide sequencing, platelets and
megakaryocytes contain substantial quantities of a protein identical to
brain and retinal Gz alpha. The paucity of Gz alpha protein and RNA in the
megakaryoblastic cell lines suggests that either there has been a selective
loss of the ability to synthesize Gz alpha from these cells or that Gz
alpha appears at a later stage in megakaryocyte development than does Gi
alpha.
Volume 78,
Issue 5,
pp. 1247-1253,
09/01/1991
Copyright © 1991 by The American Society of Hematology
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