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Evidence that the signal-initiating membrane protein CD9 is associated with small GTP-binding proteins
1Department of Medicine, Cross Cancer Institute, Edmonton, Alberta, Canada.
Insights
Antibodies targeting CD9 (a cell surface receptor) activate platelets through a pathway involving small G-proteins. This research identifies specific GTP-binding proteins associated with CD9, suggesting CD9 initiates signaling cascades.
Area of Science:
- * Molecular Biology
- * Immunology
- * Cell Signaling
Background:
- * Platelet aggregation is a critical process in hemostasis and thrombosis.
- * CD9 is a cell surface glycoprotein expressed on platelets, implicated in various cellular functions.
- * The precise signaling mechanisms initiated by CD9 engagement remain incompletely understood.
Purpose of the Study:
- * To investigate the role of CD9 as a signal-initiating molecule in platelet activation.
- * To identify proteins associated with CD9 that bind guanosine triphosphate (GTP).
- * To characterize the nature of these CD9-associated GTP-binding proteins.
Main Methods:
- * Immunoprecipitation of CD9 and associated proteins from human platelet lysates using anti-CD9 monoclonal antibodies (mAbs).
- * GTP-binding assays using [alpha 32P]GTP on nitrocellulose transfers of immunoprecipitated proteins.
- * Specificity testing of GTP-binding by assessing inhibition with GTP, adenosine triphosphate (ATP), and varying magnesium (Mg2+) concentrations.
Main Results:
- * Anti-CD9 mAbs specifically immunoprecipitated 25 and 26 kDa proteins from human platelet lysates.
- * These immunoprecipitated proteins demonstrated specific binding to [alpha 32P]GTP, which was inhibited by GTP but not ATP.
- * The GTP-binding activity was not dependent on a Mg(2+)-sensitive subset of proteins.
Conclusions:
- * CD9 is associated with specific small G-proteins in human platelets.
- * These findings support the role of CD9 as a signal-initiating molecule in platelet activation via a thromboxane-dependent pathway.
- * The identified GTP-binding proteins are likely key mediators in CD9-initiated signaling.
Abstract:
F(ab')2 fragments of anti-CD9 mAb aggregate platelets by a thromboxane-dependent pathway implicating CD9 as signal initiating molecule. We demonstrate that mAbs directed against CD9, but not against GPIIb/IIIa specifically immunoprecipitate, from detergent lysates of human platelets, proteins of 25 and 26 kDa which bind [alpha 32P]GTP on nitrocellulose transfers. The binding is specific since it is blocked by GTP, but not by ATP. The GTP-binding proteins do not belong to a Mg(2+)-sensitive subset since they are unaffected by the addition of 2 microM-20 mM Mg2+. The observations demonstrate that CD9 is associated with selected small G-proteins.