G alpha (h) (transglutaminase II) is a bifunctional enzyme possessing transglutaminase and GTPase activities. To better understand the factors affecting these two functions of G alpha (h) we have examined the characteristics of purified G alpha (h) from membrane and cytosol. GTP binding activity of mouse heart G alpha (h), was higher in membrane than that from cytosol, Furthermore, phospholipase C-delta1 (PLC-delta1) activity and coimmunoprecipitation of G alpha (h)-coupled PLC-delta1 in the alpha (1)-drenoceptor-G alpha (h)-PLC-delta1 complex preparations were increased by phenylephrine in the presence of membranous G alpha (h). On the other hand, transglutaminase activity of cytosolic G alpha (h) was higher than that from membrane G alpha (h). These results demonstrate that bifunctions of G alpha (h) are regulated by its localization that can reflect the cellular functions of G alpha (h).