Distinct pH modulation for dual function of Galphah (transglutaminase II).

Galpha(h), also known as transglutaminase II, has GTPase as well as transglutaminase activities. To better understand the factors affecting these dual enzymatic activities, we examined the optimal pH (at 25 degrees C) and thermal stability (at 37 degrees C) of the activities using membranous Galpha(h) from mouse heart. The optimum pH for the GTPase activity of Galpha(h) is approximately 7.0. As well, the GTP binding activity of Galpha(h) is more thermostable at pH 7.0 than that at pH 9.0. Consistent with these observations on the GTPase function of Galpha(h), both the phospholipase C-delta1 activity and the yield of co-immunoprecipitation of Galpha(h)-coupled phospholipase C-delta1 in alpha(1)-adrenoceptor/Galpha(h)/phospholipase C-delta1 complex preparations were enhanced by incubation with an alpha(1)-agonist, phenylephrine, at pH 7.0. On the other hand, the transglutaminase activity of Galpha(h) is higher in the basic pH range with an optimum activity at pH approximately 9.0. Also, the transglutaminase activity of Galpha(h) is more thermostable at pH 9.0 than that at pH 7.0. These results indicate not only pH as a modulator for the dual functions of Galpha(h), but also provide direct evidence for the involvement of pH in the Galpha(h)-mediated alpha(1)-adrenoceptor signaling system in vitro.