Characterization of the cAMP phosphodiesterase domain in plant adenylyl cyclase/cAMP phosphodiesterase CAPE from the liverwort Marchantia polymorpha.

Cyclic AMP (cAMP)acts as a second messenger and is involved in the regulation of various physiological responses. Recently, we identified the cAMP-synthesis/hydrolysis enzyme CAPE, which contains the two catalytic domains adenylyl cyclase (AC)and cAMP phosphodiesterase (PDE)from the liverwort Marchantia polymorpha. Here we characterize the PDE domain of M. polymorpha CAPE (MpCAPE-PDE)using the purified protein expressed in E. coli. The Km and Vmax of MpCAPE-PDE were 30 µM and 5.8 nmol min-1 mg-1, respectively. Further, we investigated the effect of divalent cations on PDE activity and found that Ca2+ enhanced PDE activity, suggesting that Ca2+ may be involved in cAMP signaling through the regulation of PDE activity of CAPE. Among the PDE inhibitors tested, only dipyridamole moderately inhibited PDE activity by approximately 40 percent at high concentrations. Conversely, 3-isobutyl-1-methylxanthine (IBMX)did not inhibit PDE activity.