Membrane topology of the cyanobacterial bicarbonate transporter, SbtA, and identification of potential regulatory loops

The transporter SbtA is a high affinity Na(+)-dependent HCO(3)(-) uptake system present in a majority of cyanobacterial clades. It functions in conjunction with CO(2) uptake systems and other HCO(3)(-) uptake systems to allow cyanobacteria to accumulate high levels of HCO(3)(-) used to support efficient photosynthetic CO(2) fixation via the CO(2) concentrating mechanism in these species. The phoA/lacZ fusion reporter method was used to determine the membrane topology of the cyanobacterial bicarbonate transporter, SbtA (predicted size of similar to 39.7 kD), cloned from the freshwater strain, Synechocystis PCC6803. The structure conforms to a model featuring 10 transmembrane helices (TMHs), with a distinct 5 + 5 duplicated structure. Both the N- and C-terminus are outside the cell and the second half of the protein is inverted relative to the first. The first putative helix appears to lack sufficient topogenic signals for its correct orientation in the membrane and instead relies on the presence of later helices. The cytoplasmic loop between helices 5 and 6 is a likely location for regulatory mechanisms that could govern activation of the transporter, and the cytoplasmic loop between helices 9 and 10 also contains some conserved putative regulatory residues.