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 (By similarity). Interacts with calmodulin. A number of isoforms are produced. According to EST sequences. Expressed in roots, inflorescence stems, flowers, siliques and leaves. Up-regulated by salt treatment (PubMed:20122158). Up-regulated by nitrogen treatments such as ammonium chloride, ammonium nitrate, glutamate and glutamine but not by potassium nitrate (PubMed:9701597). Down-regulated by hypoxia (PubMed:18077464). The C-terminus (463-494) binds calmodulin in a calcium-dependent fashion. No visible phenotype. Belongs to the group II decarboxylase family. Truncated N-terminus.