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A more recent version of this article appeared on December 1, 2003
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Submitted on April 18, 2003
Revised on July 7, 2003
Accepted on August 12, 2003
1 Department of Cell Biology and Human Anatomy, School of Medicine, University of California Davis, Davis, CA 95616, USA
2 Department of Pathology, Uniformed Services University of the Health Sciences, Bethesda, MD 20814
3 Department of Cell Biology and Human Anatomy, School of Medicine, University of California Davis, Davis, CA 95616, USA; Department of Cell Biology and Human Anatomy, School of Medicine, University of California Davis, Davis, CA 95616, USA
4 Section of Molecular and Cellular Biology, University of California Davis, Davis, CA 95616, USA
* Corresponding author. E-mail address: pdprimakoff{at}ucdavis.edu.
The function currently attributed to tetraspanins is to organize
molecular complexes in the plasma membrane using multiple
cis interactions. Additionally, the tetraspanin CD9 may
be a receptor which binds the soluble ligand PSG17, a member of the
immunoglobulin superfamily (IgSF)/CEA subfamily. However, previous data
are also consistent with the PSG17 receptor being a CD9
cis-associated protein. In the current study, CD9
extracellular loop (EC2) specifically bound to PSG17-coated beads,
indicating a direct interaction between the two proteins. However,
CD9-EC2 did not bind to PSG17-coated beads if the CD9-EC2 had the
mutation SFQ (173-175) to AAA, a previously studied mutation in egg
CD9 that abolishes sperm-egg fusion. Also PSG17 bound to 293 T cells
transfected with wild-type CD9 but not the mutant CD9. By
immunofluorescence PSG17 bound to wild-type eggs but not to CD9 null
eggs. The presence of 
2 µM recombinant PSG17 produced a
significant and reversible inhibition (60-80%) of sperm-egg fusion.
Thus, we conclude that CD9 is a receptor for PSG17 and when the PSG17
binding site is mutated or occupied, sperm-egg fusion is impaired.
These findings suggest that egg CD9 may function in gamete fusion by
binding to a sperm IgSF/CEA subfamily member and such proteins have
previously been identified on sperm.
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