YM-254890, which was isolated from the culture broth of Chromobacterium sp., inhibits ADP-induced platelet aggregation and has antithrombotic and thrombolytic effects. YM-254890 blocks Galpha(q/11)-coupled ADP receptor P2Y1-mediated Ca(2+) mobilization. Here we report that YM-254890 is a selective Galpha(q/11) inhibitor. YM-254890 blocked Ca(2+) mobilization mediated by several Galpha(q/11)-coupled receptors but not by Galpha(i)- or Galpha(15)-coupled receptor, indicating that phospholipase Cbeta activation and subsequent signaling molecules are not the target of YM-254890. YM-254890 completely prevented the serum response factor (SRF)-mediated gene transcription induced by Galpha(q)R183C, which is constitutively active in a receptor-dependent manner because of its reduced k(cat) of GTP hydrolysis. Conversely, YM-254890 had only a modest effect on the SRF-mediated gene transcription by Galpha(q)Q209L, which is GTPase-deficient (activated) Galpha(q). These suggested that the acting point of YM-254890 is receptor-Galpha(q) interaction or the subsequent guanine nucleotide exchange step. The fact that YM-254890 (i) inhibited the SRF-mediated gene transcription by Galpha(qi5), which interacts with Galpha(i)-coupled receptor and possesses the effector function of Galpha(q), and (ii) had no effect on the K(d) value of high affinity [(3)H]2MeSADP binding to P2Y1, which reflects the agonist-receptor-Galpha ternary complex, suggested that receptor-Galpha(q/11) interaction is not the target of YM-254890. On the other hand, specific [(35)S]GTPgammaS binding to Galpha(q/11) stimulated by the M1 muscarinic acetylcholine receptor and P2Y1 were inhibited by YM-254890. These data indicate that YM-254890 blocks the exchange of GDP for GTP in Galpha(q/11) activation. This novel Galpha(q/11)-selective inhibitor is a promising and powerful tool for studying Galpha(q/11) protein activation, Galpha(q/11) -coupled receptor signaling, and Galpha(q/11)-mediated biological events.