Rap1GAP2 is a new GTPase-activating protein of Rap1 expressed in human platelets

The Ras-like guanine-nucleotide-binding protein Rap1 controls integrin 04103 activity and platelet aggregation. Recently, we have found that Rapt activation can be blocked by the nitric oxide/cyclic guanosine monophosphate (NO/cGMP) signaling pathway by type 1 cGMP-dependent protein kinase (cGKI). in search of possible targets of NO/cGMP/cGKI, we studied the expression of Rap1-specific GTPase-activating proteins (GAPs) and guanine nucleotide exchange factors (GEFs) in Platelets. We could detect mRNAs for a new protein most closely related to Rap1 GAP and for postsynaptic density-95 discs-large and zona occludens protein 1 (PDZ)-GEF1 and CalDAG-GEFs I and III. Using 5'-rapid amplification of cDNA ends ( RACE), we isolated the complete cDNA of the hew GAP encoding a 715-amino acid protein, which we have termed Rapt GAP2. Rapt GAP2 is expressed in at least 3 splice variants, 2 of which are detectable in platelets. Endogenous Rap1GAP2 protein partially colocalizes with Rapt in human platelets. In transfected cells, we show that Rap1GAP2 exhibits strong GTPase-stimulating activity toward Rap1. Rap1GAP2 is highly phosphorylated, and we have identified cGKI as a Rap1GAP2 kinase. cGkI phosphorylates Rapt GAP2 exclusively on serine 7, a residue present only in the platelet splice variants of Rapt GAP2. Phosphorylation of Rap1GAP2 by cGKI might mediate inhibitory effects of NQ/cGMP on Rap1. Rap1GAP2 is the first GTPase-activating protein of Rapt found in platelets and is likely to have an important regulatory role in platelet aggregation. (c) 2005 by The American Society of Hematology.