Evidence that the signal-initiating membrane protein CD9 is associated with small GTP-binding proteins.

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.