Activation of a recombinant petunia glutamate decarboxylase by calcium/calmodulin or by a monoclonal antibody which recognizes the calmodulin binding domain

To date, only plants have been shown to possess a form of glutamate decarboxylase (GAD) that binds calmodulin. In the present study, a recombinant calmodulin-binding 58-kDa petunia GAD produced in Escherichia coli was purified to homogeneity using calmodulin-affinity chromatography, and its responsiveness to calcium and calmodulin was examined in vitro. At pH 7.0-7.5, the purified recombinant enzyme was essentially inactive in the absence of calcium and calmodulin, but it could be stimulated to high levels of activity (V(max) = 30 μmol of CO₂ min^-1 mg of protein^-1) by the addition of exogenous calmodulin (K_0.5 = 15 nM) in the presence of calcium (K_0.5 = 0.8 μM). Neither calcium nor calmodulin alone had any effect on GAD activity. Recombinant GAD displayed hyperbolic kinetics at pH 7.3 (K(m) = 8.2 mM). A monoclonal antibody directed against the carboxyl-terminal region, which contains the calmodulin-binding domain of GAD, was able to fully activate GAD in a dose-dependent manner in the absence of calcium and calmodulin, whereas an antibody recognizing an epitope outside of this region was unable to activate GAD. This study provides the first evidence that the activity of the purified 58-kDa GAD polypeptide is essentially calcium/calmodulin-dependent at physiological pH. Furthermore, activation of GAD by two different proteins that interact with the calmodulin-binding domain, a monoclonal antibody or calcium/calmodulin, suggests that this domain plays a major role in the regulation of plant GAD activity.