Expression of GAPs and GEFs of Rap1 in human platelets. RT-PCR analysis of GAP (A) and GEF (B) expression in washed human platelets was performed using transcript-specific primers for reverse transcription and PCR. As positive control, human brain RNA was used in most reactions except SPA-1, KIAA1039, KIAA0545, and CalDAG-GEFI (CD-GEF I), which were detected in spleen (bottom row of each panel). Results shown were confirmed using oligo-(d)T priming for first-strand synthesis. None of the known GAPs could be detected in platelets. Database searches disclosed a number of cDNAs encoding new potential Rap1GAPs; of these, KIAA1039 was expressed in platelets. The only GEFs of Rap1 detectable in platelets were CalDAG-GEFI (CD-GEF I)/RasGRP2, CalDAG-GEFIII (CD-GEF III)/RasGRP3, and PDZ-GEF1. Shown are representative results from at least 3 (platelets) or 2 (control) similar experiments.