The alpha(1)-adrenergic receptors activate a phospholipase C enzyme by coupling to members of the large molecular size (approximately 74 to 80 kilodaltons) G alpha(h) family of guanosine triphosphate (GTP)-binding proteins. Rat liver G alpha(h) is now shown to be a tissue transglutaminase type II (TGase II). The transglutaminase activity of rat liver TGase II expressed in COS-1 cells was inhibited by the nonhydrolyzable GTP analog guanosine 5’-O-(3-thiotriphosphate) or by alpha(1)-adrenergic receptor activation. Rat liver TGase II also mediated alpha(1)-adrenergic receptor stimulation of phospholipase C activity. Thus, G alpha(h) represents a new class of GTP-binding proteins that participate in receptor signaling and may be a component of a complex regulatory network in which receptor-stimulated GTP binding switches the function of G alpha(h) from transglutamination to receptor signaling.