The monoclonal antibody AG-1, a potent stimulator of human platelets, interacts with a low molecular weight GTP-binding protein.
AG-1 is a monoclonal antibody that binds to human platelets and causes aggregation and secretion. Previous work has established that these responses result from phospholipase C-mediated hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2). To determine the mechanism by which this ligand induces signals for platelet activation, we performed a series of experiments examining the platelet binding site for AG-1. AG-1 immunoprecipitates from radioiodinated human platelet plasma membranes a protein of Mr 21,000. AG-1 immunoprecipitated proteins separated by SDS-PAGE, transferred to nitrocellulose, and incubated with [alpha 32P]GTP demonstrate binding of the radiolabeled GTP to the Mr 21,000 protein. A 100-fold molar excess of unlabeled GTP inhibits completely this binding of [alpha 32P]GTP. These results indicate that AG-1 interacts with a low Mr GTP-binding protein on the surface of platelets and suggests that either the protein recognized by AG-1 or a coprecipitating molecule of similar Mr is a low Mr GTP-binding protein that may function in platelet extracellular signal transduction.
Kroll
M H
MH
Hematology Section, Houston VA Medical Center, TX.
Claure
R E
RE
Miller
J L
JL
D000911:Antibodies, Monoclonal; D001792:Blood Platelets / Q000276:immunology / Q000502:physiology*; D019204:GTP-Binding Proteins / Q000097:blood* / Q000276:immunology; D006160:Guanosine Triphosphate / Q000378:metabolism; D006801:Humans; D007700:Kinetics; D010974:Platelet Aggregation; D015398:Signal Transduction
D000911:Antibodies, Monoclonal; D006160:Guanosine Triphosphate; D019204:GTP-Binding Proteins