title: Topology mapping to characterize cyanobacterial bicarbonate transporters: BicA (SulP/SLC26 family) and SbtA.
abstract: This mini-review addresses advances in understanding the transmembrane topologies of two unrelated, single-subunit bicarbonate transporters from cyanobacteria, namely BicA and SbtA. BicA is a Na(+)-dependent bicarbonate transporter that belongs to the SulP/SLC26 family that is widespread in both eukaryotes and prokaryotes. Topology mapping of BicA via the phoA/lacZ fusion reporter method identified 12 transmembrane helices with an unresolved hydrophobic region just beyond helix 8. Re-interpreting this data in the light of a recent topology study on rat prestin leads to a consensus topology of 14 transmembrane domains with a 7+7 inverted repeat structure. SbtA is also a Na(+)-dependent bicarbonate transporter, but of considerably higher affinity (Km 2-5 μM versus >100 μM for BicA). Whilst SbtA is widespread in cyanobacteria and a few bacteria, it appears to be absent from eukaryotes. Topology mapping of SbtA via the phoA/lacZ fusion reporter method identified 10 transmembrane helices. The topology consists of a 5+5 inverted repeat, with the two repeats separated by a large intracellular loop. The unusual location of the N and C-termini outside the cell raises the possibility that SbtA forms a novel fold, not so far identified by structural and topological studies on transport proteins.
authors.affiliation: Molecular Plant Physiology Cluster, Plant Science Division and.
authors.forename: G Dean
authors.initials: GD
authors.lastname: Price
authors.forename: Susan M
authors.initials: SM
authors.lastname: Howitt
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry* / Q000235:genetics; D001639:Bicarbonates / Q000378:metabolism*; D000458:Cyanobacteria / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D016623:Ion Pumps / Q000737:chemistry* / Q000235:genetics; D008958:Models, Molecular; D010449:Peptide Mapping; D017433:Protein Structure, Secondary
chemical_list: D001426:Bacterial Proteins; D001639:Bicarbonates; D016623:Ion Pumps
keywords: BicA; bicarbonate transporters; membrane proteins; sulp/SLC26 SbtA; topology