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Vol. 14, Issue 12, 5098-5103, December 2003

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Direct Binding of the Ligand PSG17 to CD9 Requires a CD9 Site Essential for Sperm-Egg Fusion

Diego A. Ellerman ^* , Cam Ha  , Paul Primakoff ^* , Diana G. Myles ^||, and Gabriela S. Dveksler 

^* Department of Cell Biology and Human Anatomy, School of Medicine, University of California Davis, Davis, California 95616; Department of Pathology, Uniformed Services University of the Health Sciences, Bethesda, Maryland 20814; and ^|| Section of Molecular and Cellular Biology, University of California Davis, Davis, California 95616

Submitted April 18, 2003; Revised July 7, 2003; Accepted August 12, 2003
Monitoring Editor: Mary Beckerle

The function currently attributed to tetraspanins is to organize molecular complexes in the plasma membrane by using multiple cis-interactions. Additionally, the tetraspanin CD9 may be a receptor that 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.

Corresponding author. E-mail address: pdprimakoff{at}ucdavis.edu.

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