Sodium-dependent bicarbonate transporter SbtA–SbtB complex (SbtAB transporter)

The SbtAB transporter complex is composed of SbtA, a membrane-embedded sodium-dependent high-affinity bicarbonate transporter, and SbtB, a cytoplasmic PII-like regulatory protein. SbtA facilitates the symport of sodium ions and bicarbonate anions into cyanobacterial cells as part of the CO₂-concentrating mechanism (CCM), critical for photosynthetic carbon fixation. SbtB binds to the cytoplasmic portion of SbtA, allosterically inhibiting its activity via nucleotide binding, particularly AMP, thus controlling cellular bicarbonate uptake in response to metabolic state. Structurally, SbtA belongs to the Na^+/solute symporter family (TC.2.A.83), containing 10 transmembrane helices organized into core (substrate-binding) and gate (scaffold) domains. Transport occurs via an "elevator mechanism," where the core domain moves as a rigid body between inward- and outward-facing states. The trimeric configuration is the functional unit, with each SbtB subunit interacting with one SbtA subunit to modulate activity. SbtA's function is essential in cyanobacteria for efficient carbon fixation. While not a therapeutic target in humans, improving or modifying SbtAB activity in plants could benefit agricultural crop engineering. No drugs or clinical biomarkers are relevant to this system, and no safety concerns have been reported since SbtAB research is confined to cyanobacteria and synthetic biology.

Other names

SbtASbtBSbtAB (the heterocomplex of SbtA and SbtB)cyanobacterial bicarbonate transporterTC.2.A.83 family transporter

Not applicable; transporter activity is inhibited allosterically by SbtB binding to SbtA in presence of adenyl nucleotides (e.g., AMP). Theoretical mechanisms for modulation may involve blocking substrate-binding or altering allosteric regulation.