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in orientation was interpreted as related to whether the intracellular domains of
EPOR would be brought into productive or non-productive orientations.
Results from our laboratory highlighted the productive or non-productive
orientations of EPOR dimer. A conserved hydrophobic motif (L253, I257,
W258) in EPOR juxtamembrane cytoplasmic region just
N
-terminal to the
N
conserved Box 1 motif is necessary for receptor signaling [22]. Using a com-
bination of computer modeling and mutagenesis, we showed that this motif is
conformationally rigid and is contained within an
-helix continuous with the
α
transmembrane
-helix. An insertional mutation of three alanines, which
maintains the register of the
α
-helix encompassing the hydrophobic motif,sig-
nals like wild-type EPOR. Alteration of the register of the
α
-helix by inserting
one alanine results, however, in a change in JAK-2 positioning on EPOR. In
this case, EPO induces JAK-2 transphosphorylation but these kinases are posi-
tioned in a way that they cannot phosphorylate EPOR and are inactive in
EPOR signaling.
α
R129C
A constitutively active form of EPOR, R129C, was isolated by its ability to
support EPO-independent proliferation of Ba/F3 cells [14]. Unlike the wild-
type EPOR, this mutant forms disulfide-linked homodimers in the absence of
EPO [15].
Mice infected with a virus carrying the R129C mutant EPOR develop poly-
cythemia and splenomegaly, demonstrating that R129C stimulates expansion
of the erythroid compartment
in vivo
, with a concomitant increase in the num-
ber of circulating erythrocytes [27]. When factor-independent cells were iso-
lated from the spleen of the infected mice and injected into syngenetic mice, a
fatal leukemia developed with a massive expansion of erythroblast-like cells
that populated the bone marrow and spleen of the recipient animals [27].
R129C also stimulates EPO-independent development of CFU-E cells into
mature erythrocytes
in vitro
[28]. Artificial overexpression of R129C EPOR
mutant in different lineages induces leukemia, suggesting that once expressed,
EPOR generates signals that induce proliferation of many types of progenitors
additional to erythroid progenitors [27]. This report described the first onco-
genic mutation in a member of the cytokine-receptor family.
Friend virus-induced erythroleukemia
Friend virus consists of a replication-competent murine leukemia virus
(FMuLV) and a replication-defective spleen focus-forming virus (SFFV).
Genetic studies have shown that the transmembrane protein gp55 of the defec-
tive SFFV is responsible for Friend virus-induced erythroleukemia [29]. The
two known strains of SFFV, the anemic or “A” strain and the polycythemic or
