Carbonic Anhydrases Fused to Anion Transporters of the SulP Family: Evidence for a Novel Type of Bicarbonate Transporter

The sulfate permease (SulP) family of secondary carriers (TC #2.A.53) includes functionally characterized members that are inorganic anion:H+ symporters and anion:anion antiporters. We here describe members of this family that are fused to non-transporter domains, a relatively rare occurrence in prokaryotes. One subfamily includes members that are either fused to or are encoded within operons that also encode homologues of carbonic anhydrases, suggesting that these carriers function to take up bicarbonate or carbonate. Within another subfamily, a SulP homologue is fused to rhodanese, a thiosulfate:cyanide sulfotransferase, suggesting that this carrier functions in sulfate uptake. Some homologues are encoded in operons that also encode putative Na+/H+ antiporters of the NhaD family (TC #2.A.62) or putative Na+:HCO3- symporters of the SBT family (TC #2.A.83). SulP homologues present in fungi and some bacteria are fused to cyclic AMP-binding domains and STAS domains that presumably function in regulation or targeting. Phylogenetic analyses reveal the relationships of these proteins and protein domains to each other and show that in some cases, but not in others, the hydrophilic domains/proteins have coevolved with the transporters.