Biological functions of the unusual guanine nucleotide-binding protein Gαh: transglutaminase II

Regulation of cellular response to external stimuli, hormones and neurotransmitters, is an important mechanism for controlling cellular functions. The transmembrane signaling of the hormone

receptors is regulated by GTP-binding proteins and their associated proteins. A new class of GTP-binding protein, Gαh is a bifunctional enzyme possessing two biological functions, namely

GTPase and transglutaminase (TGase) activity. This bifunctional Gαh mediates the α1B-adrenoceptor signal to an 85- kDa phospholipase C-δ1 (PLC-δ1) and associates with a -50 kDa protein (Gβh)

which regulates the the GTP/GDP binding affinity of Gαh. Gh, the holoenzyme is thus a heterodimer, and the subunits dissociate from each other upon activation with GTP or its analogues. The

GTPase and TGase activities of Gαh are regulated by two reciprocal activators, Ca2+ and GTP. The GTP-binding subunit, Gαh directly interacts with the α1B-adrenoceptor and stimulates PLC-δ1.

Gβh, however, neither interacts with the receptor nor stimulates PLC-δ1. Therefore, Gβh functions as a counterpart to the receptor for the activation of Gαh.

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