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Blood, 1 March 2002, Vol. 99, No. 5, pp. 1683-1691
IMMUNOBIOLOGY
Cell surface antigen CD109 is a novel member of the
 2 macroglobulin/C3, C4, C5 family of
thioester-containing proteins
Martin Lin,
D. Robert Sutherland,
Wendy Horsfall,
Nicholas Totty,
Erik Yeo,
Rakash Nayar,
Xiang-Fu Wu, and
Andre C. Schuh
From the Departments of Medical Biophysics, and
Medicine, and the Institute of Medical Science, University of Toronto;
the Division of Hematology/ Medical Oncology, The Princess Margaret
Hospital, Toronto, ON, Canada; and the Imperial Cancer Research Fund,
London, United Kingdom.
Cell surface antigen CD109 is a glycosylphosphatidylinositol
(GPI)-linked glycoprotein of approximately 170 kd found on a subset of hematopoietic stem and progenitor cells and on activated platelets and T cells. Although it has been suggested that T-cell CD109
may play a role in antibody-inducing T-helper function and it is known
that platelet CD109 carries the Gov alloantigen system, the role of
CD109 in hematopoietic cells remains largely unknown. As a first step
toward elucidating the function of CD109, we have isolated and
characterized a human CD109 cDNA from KG1a and endothelial cells. The
isolated cDNA comprises a 4335 bp open-reading frame encoding a 1445 amino acid (aa) protein of approximately 162 kd that contains a 21 aa
N-terminal leader peptide, 17 potential N-linked glycosylation sites,
and a C-terminal GPI anchor cleavage-addition site. We report that
CD109 is a novel member of the  2 macroglobulin (2M)/C3,
C4, C5 family of thioester-containing proteins, and we demonstrate that
native CD109 does indeed contain an intact thioester. Analysis of the
CD109 aa sequence suggests that CD109 is likely activated by
proteolytic cleavage and thereby becomes capable of thioester-mediated
covalent binding to adjacent molecules or cells. In addition, the
predicted chemical reactivity of the activated CD109 thioester is
complementlike rather than resembling that of 2M proteins.
Thus, not only is CD109 potentially capable of covalent binding to
carbohydrate and protein targets, but the t1/2
of its activated thioester is likely extremely short, indicating that
CD109 action is highly restricted spatially to the site of its activation.
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