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