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Blood, Vol. 92 No. 8 (October 15), 1998:
pp. 2940-2950
Molecular Identification and Functional Characterization of a Novel
Protein That Mediates the Attachment of Erythroblasts to
Macrophages
Manjit Hanspal,
Yva Smockova, and
Quang Uong
From the Department of Biomedical Research, St Elizabeth's Medical
Center, Tufts University School of Medicine, Boston, MA.
We have previously identified a novel protein that mediates the
attachment of erythroblasts to macrophages in vitro. This attachment
promotes terminal maturation and enucleation of erythroblasts (Hanspal
and Hanspal, Blood 84:3494, 1994). This protein is referred to
here as Emp for erythroblast macrophage
protein. Two immunologically related isoforms of Emp with
apparent molecular weights of 33 kD and 36 kD were detected in
macrophage membranes. The complete amino acid sequence of the larger
isoform of Emp was deduced from the nucleotide sequence of a
full-length 2.0-kb cDNA that was isolated from a human macrophage cDNA
library using affinity-purified anti-Emp antibodies. Of the 2,005 bp,
1,185 bp encode for 395 amino acids representing 43 kD (the sodium
dodecyl sulfate-polyacrylamide gel electrophoresis [SDS-PAGE]
molecular mass is 36 kD). Northern blot analysis of human macrophage
poly(A) RNA detected a message for Emp of 2.1 kb. The deduced amino
acid sequence contains a putative transmembrane domain near the
N-terminus. To investigate the structure/function relationships of Emp,
recombinant fusion proteins of full-length and truncated Emp were
produced in bacteria, COS-7, and HeLa cells. Cell binding assays showed
that the N-terminus is exposed on the cell surface. The recombinant Emp
functions as a cell attachment molecule when expressed in heterologous
cells. Furthermore, we showed that the demise of erythroblasts in the absence of Emp-mediated erythroblast-macrophage association is accompanied by apoptosis. We postulate that Emp-mediated contact between erythroblasts and macrophages promotes terminal maturation of
erythroid cells by suppressing apoptosis.
© 1998 by The American Society of Hematology.
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