9.B.67 The Putative Inorganic Carbon (HCO3-) Transporter/O-antigen Polymerase (ICT/OAP) Family
Many photosynthetic organisms including cyanobacteria can concentrate CO2/HCO3- against a greater than ten-fold concentration gradient. This is the difference observed for dissolved CO2 at equilibrium in air and the KM for CO2 of ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco). In cyanobacteria, the CO2 concentrating mechanism involves (1) CO2 conversion to HCO3-, (2) energy-dependent HCO3- transport, and (3) carbonic anhydrase-catalyzed formation of CO2 from HCO3- in close proximity to Rubisco in the carboxysomes. The gene believed to encode the transporter, possibly a Na+:HCO3- symporter, is designated ictB or ORF467 in Synechococcus sp. strain PCC7942. The protein is 467 amino acyl residues long and possesses 10-12 putative transmembrane α-helical spanners. PSI-BLAST searches of the databases revealed homology with members of the O-antigen polymerase (ligase) family (COG3307). Although the authors claim that IctB is 'highly homologous to several transport-engaged and oxidation-reduction proteins including the Na+:pantothenate symporter of E. coli' (of the SSS family; TC #2.A.21), no significant similarity could be documented. It seems likely that these homologues are O-antigen polymerases instead of bicarbonate transporters. It is possible that O-antigen polymerases facilitate transport of intermediates of LPS biosynthesis. The four proteins listed in this table are very distantly related to each other, but homology has been established (DE Chen and MH Saier, unpublished results). This family may be distantly related to the HHP (9.B.14) and APC (2.A.3) Families. This family may be distantly related to 9.B.128, also designated as an O-antigen polymerase family.
HCO3- uptake in cyanobacteria is Na+-dependent, and a Na+ symport mechanism has been proposed. The probable transport reaction proposed for IctB is therefore:
HCO3- (out) + Na+ (out) → HCO3- (in) + Na+ (in).