Catalyzes the conversion of glutamate to 4-aminobutanoate (GABA). The calmodulin-binding is calcium-dependent and it is proposed to directly or indirectly form a calcium regulated control of GABA biosynthesis. H(+) + L-glutamate = 4-aminobutanoate + CO2 Up-regulated by calmodulin binding at physiological pH. Optimum pH is 6.0. Homohexamer. Interacts with calmodulin with a 1:3 stoichiometry. Expressed in roots. Detected at low levels in shoots of young seedlings. Not detected in the root tips or in the central vascular bundle in the elongating region of mature roots. Down-regulated by salt treatment. Not induced by hypoxia. The N-terminus (1-57) is involved in the formation of the multimer. The C-terminus (471-502) binds calmodulin in a calcium-dependent fashion and contains probably an autoinhibitory domain. No visible phenotype, but increased glutamate levels and decreased GABA levels in the roots, and loss of GABA accumulation upon heat stress. Belongs to the group II decarboxylase family.