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 at low levels in siliques. Belongs to the group II decarboxylase family.