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J. Biol. Chem., Vol. 276, Issue 44, 41518-41525, November 2, 2001
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From the Department of Molecular and Cellular Biology, Harvard
University, Cambridge, Massachusetts 02138
CD39 is a member of the membrane-bound
ecto-nucleoside triphosphate diphosphohydrolase family. The active site
for native CD39 is located on the outer surface of the cellular plasma
membrane; however, it is not yet known at what stage this enzyme
becomes active along the secretory pathway to the plasma membrane. In this study, sucrose density fractionations performed on
CD39-transfected COS-7 cell membranes suggest that CD39 activity
resides primarily in the plasma membrane. Furthermore, we have created
recombinant, soluble versions of CD39, one that is secreted and others
that are retained in the endoplasmic reticulum, to demonstrate
that CD39 is not active until it reaches the plasma membrane both in yeast and COS-7 cells. Moreover, the secreted active soluble CD39 in
COS-7 cells is found to receive a higher degree of N-glycan addition than the inactive form retained intracellularly. When COS-7
cells were treated with tunicamycin to prevent
N-glycosylation, soluble CD39 was not detected in the
extracellular medium and remained inactive intracellularly. Surface
biotinylation analysis also revealed that surface-expressed wild type
CD39 receives a higher degree of N-glycosylation than
intracellular forms and that inhibition of N-glycosylation
prevents its plasma membrane localization. In addition, both intact and
digitonin-permeablized COS-7 cells transfected with CD39 possess
similar ecto-ATPase activities, further supporting the conclusion that
only surface-expressed CD39 is enzymatically active. All of these data
suggest that intracellular CD39 is inactive and that only a fully
glycosylated CD39 has apyrase activity and is localized at the cell surface.
To whom correspondence should be addressed: Dept. of Molecular & Cellular Biology, Harvard University, 7 Divinity Ave., Cambridge, MA
02138. Tel.: 617-495-2301; Fax: 617-495-8308; E-mail:
guidotti@fas.harvard.edu.
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