Generates GABA from glutamate. GABA induces the release of acbA from prespore cells and induces the exposure of tagC on the surface of prestalk cells where it can convert acbA to SDF-2. Glutamate acts as a competitive inhibitor. H(+) + L-glutamate = 4-aminobutanoate + CO2 Homohexamer. Exclusively expressed in prespore cells late in development. Expressed only after 10 hours of development and its mRNA accumulates to peak at 18 hours when cells are initiating fruiting body formation. Highly expressed during phagocytosis of non-pathogenic bacteria. Mutant cells grow and develop well, forming normally proportioned fruiting bodies, but the number of viable spores is reduced to about half. Belongs to the group II decarboxylase family.