Converts internalized glutamate to GABA and increases the internal pH. Involved in glutamate-dependent acid resistance. H(+) + L-glutamate = 4-aminobutanoate + CO2 Expression is highest at onset of stationary phase in presence of NaCl and glutamate, and at low pH. Chloride-dependent expression is activated by GadR. Belongs to the group II decarboxylase family.