Decarboxylase subunit of the primary sodium pump glutaconyl-CoA decarboxylase (GCD). (2E)-glutaconyl-CoA + H(+) + Na(+)(in) = (2E)-butenoyl-CoA + CO2 + Na(+)(out) Amino-acid degradation; L-glutamate degradation via hydroxyglutarate pathway; crotonoyl-CoA from L-glutamate: step 5/5. Heterooctamer consisting of two alpha, two beta, two gamma and two delta subunits.