2.A.83 The Na+-dependent Bicarbonate Transporter (SBT) Family
Cyanobacteria possess a Na+-dependent bicarbonate (HCO3-) uptake system that is induced when the CO2 concentration is low (Shibata et al., 2002). The protein that catalyzes uptake of HCO3- is SbtA (slr1512) of Synechocystis PCC 6803. It is 374 aas long and possesses 10 putative TMSs which probably arose by an intragenic duplication event where the primordial gene encoded a 5 TMS element. Inactivation of a homologue of NtpJ of Enterococcus hirae (TC #2.A.38.4.1) (slr1509), which appears to function with SbtA, prevents growth of the bacteria in the presence of high Na+ (>100 mM). This suggests that slr1509 is a Na+/H+ antiporter that in part generates the sodium motif force (smf) required for the uptake of HCO3- via the SbtA protein.
There are two types of SbtA homologues in cyanobacteria, but all appear to function by a mechanism dependent on an NtpJ homologue (Shibata et al., 2002). SbtA and its homologues display a common topology of ten transmembrane segments (TMSs). These proved to have arisen by an intragenic duplication event from an ancestral gene encoding a five TMS protein product (von Rozycki et al., 2004). A region of SbtA shows sufficient similarity to 10 TMS ABC-type integral membrane transport proteins to suggest a common origin. Phylogenetic analyses of the SbtA family revealed two clusters of cyanobacterial homologues with all non-cyanobacterial family members outside of these two clusters. The tree topology suggests that SbtA family members display multiple transport functions (von Rozycki et al., 2004).
The transport reaction catalyzed by SbtA is:
HCO3- (out) + nNa+ (out) → HCO3- (in) + nNa+ (in)