Bicarbonate binding activity of the CmpA protein of the cyanobacterium synechococcus sp. strain PCC 7942 involved in active transport of bicarbonate

The cmpABCD operon of the cyanobacterium Synechococcus sp. strain PCC 7942 encodes an ATP-binding cassette transporter involved in HCO₃^- uptake. The three genes, cmpBCD, encode membrane components of an ATP-binding cassette transporter, whereas cmpA encodes a 42-kDa cytoplasmic membrane protein, which is 46.5% identical to the membrane-anchored substrate-binding protein of the nitrate/nitrite transporter. Equilibrium dialysis analysis using H¹⁴CO₃^- showed that a truncated CmpA protein lacking the N-terminal 31 amino acids, expressed in Escherichia coli cells as a histidine-tagged soluble protein, specifically binds inorganic carbon (CO₂ or HCO₃^-). The addition of the recombinant CmpA protein to a buffer caused a decrease in the concentration of dissolved CO₂ because of the binding of inorganic carbon to the protein. The decrease in CO₂ concentration was accelerated by the addition of carbonic anhydrase, indicating that HCO₃^-, but not CO₂, binds to the protein. Mass spectrometric measurements of the amounts of unbound and bound HCO₃^- in CmpA solutions containing low concentrations of inorganic carbon revealed that CmpA binds HCO₃^- with high affinity (K(d) = 5 μM). A similar dissociation constant was obtained by analysis of the competitive inhibition of the CmpA protein on the carboxylation of phosphoenolpyruvate by phosphoenolpyruvate carboxylase at limiting concentrations of HCO₃^-. These findings showed that the cmpA gene encodes the substrate-binding protein of the HCO₃^- transporter.