Rap1A is a low molecular weight guanosine triphosphate (GTP)-binding protein in human neutrophil membranes whose cellular function is unknown. Rap1A was found to form stoichiometric complexes with the cytochrome b558 component of the phagocyte nicotinamide adenine dinucleotide phosphate (NADPH) oxidase system. The (guanosine-5′-O-(3-thiotriphosphate) (GTP-γ-S)-bound form of Rap1A bound more tightly to cytochrome b 558 than did the guanosine diphosphate-bound form. No complex formation was observed between cytochrome b558 and H-Ras-GTP-γ-S or Rap1A-GTP-γ-S that had been heat-inactivated, nor between Rap1A-GTP-γ-S and hydrophobic proteins serving as controls. Complex formation between Rap1A-GTP-γ-S and cytochrome b558 was inhibited by phosphorylation of Rap1A with cyclic adenosine monophosphate (cAMP)-dependent protein kinase. These observations suggest that Rap1A may participate in the structure or regulation of the NADPH oxidase system and that this function of the Rap1A protein may be altered by phosphorylation.
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