Converts internalized glutamate to GABA and increases the internal pH. Involved in glutamate-dependent acid resistance (By similarity). H(+) + L-glutamate = 4-aminobutanoate + CO2 Optimum pH is 4.7. Active at acidic pHs but inactive in the neutral pH range. Active up to 70 degrees Celsius at pH 4.1. Belongs to the group II decarboxylase family.