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Reg lation of cellular response to external stimuli, h.)rruones and neurotransmitters, is an important eclanism for controlling cellular functions. The Iran membrane signaling of the hormone receptors is rc gulated by GTP-binding proteins and their assn ciated proteins. A new class of GTP-binding prof !in, Gah is a bifunctional enzyme possessing two biological functions, namely GTPase and tran! tglutaminase (TGase) activity. This bifunctional Gah mediates the a1B-adrenoceptor signal to an 85-kDa phospholipase C-81 (PLC-51) and associates Witt a --50 kDa protein (G01h) which regulates the the GTP/GDP binding affinity of Gah. Gh, the hol renzyme is thus a heterodimer, and the suk units dissociate from each other upon acti-ation with GTP or its analogues. The GTPase and TGase activities of Gah are regulated by two rec procal activators, Cat+ and GTP. The GTP-bin ing subunit, Gah directly interacts with the a1B-adr, noceptor and stimulates PLC-81. Gah, however, neit ier interacts with the receptor nor stimulates PL( -81. Therefore, GRh functions as a counterpart to t e receptor for the activation of Gα(_h).
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