Beta subunit of the biotin-dependent malonate decarboxylase multienzyme complex (EC 7.2.4.4). Acts as an integral membrane-bound carboxybiotin protein decarboxylase by releasing the carboxyl group of the carboxylated biotin carrier MADF. The free energy of the decarboxylation reaction is used to pump Na(+) out of the cell. H(+) + N(6)-carboxybiotinyl-L-lysyl-[protein] + n Na(+)(in) = CO2 + N(6)-biotinyl-L-lysyl-[protein] + n Na(+)(out)