Catalyzes the production of GABA. The calmodulin-binding is calcium-dependent and it is proposed that this may, directly or indirectly, form a calcium regulated control of GABA biosynthesis (By similarity). H(+) + L-glutamate = 4-aminobutanoate + CO2 Homohexamer. Interacts with calmodulin (By similarity). Expressed in flowers and shoots. Detected at low levels in siliques, stems, leaves and roots. Up-regulated by salt treatment and hypoxia. Belongs to the group II decarboxylase family.