Direct binding of the ligand PSG17 to CD9 requires a CD9 site essential for sperm-egg fusion.
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 approximately 2 microM 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.
Ellerman
Diego A
DA
Department of Cell Biology and Human Anatomy, School of Medicine, University of California Davis, Davis, California 95616, USA.
Ha
Cam
C
Primakoff
Paul
P
Myles
Diana G
DG
Dveksler
Gabriela S
GS
D000818:Animals; D015703:Antigens, CD / Q000378:metabolism* / Q000502:physiology; D002478:Cells, Cultured; D005260:Female; D005455:Fluorescent Antibody Technique; D006801:Humans; D007136:Immunoglobulins / Q000378:metabolism*; D008297:Male; D008562:Membrane Glycoproteins / Q000378:metabolism* / Q000502:physiology; D051379:Mice; D008813:Mice, Inbred ICR; D009154:Mutation; D009865:Oocytes / Q000378:metabolism* / Q000502:physiology; D011485:Protein Binding / Q000502:physiology; D017434:Protein Structure, Tertiary / Q000502:physiology; D013081:Sperm Motility / Q000502:physiology; D013084:Sperm-Ovum Interactions / Q000502:physiology*; D013094:Spermatozoa / Q000378:metabolism* / Q000502:physiology; D060245:Tetraspanin 29
D015703:Antigens, CD; C554869:CD9 protein, human; C554870:Cd9 protein, mouse; D007136:Immunoglobulins; D008562:Membrane Glycoproteins; D060245:Tetraspanin 29