Structural Basis for the Allosteric Regulation of the SbtA Bicarbonate Transporter by the PII-like Protein, SbtB, from Cyanobium sp. PCC7001

Cyanobacteria have evolved a suite of enzymes and inorganic carbon (C i ) transporters that improve photosynthetic performance by increasing the localized concentration of CO 2 around the primary CO 2 -fixating enzyme, Rubisco. This CO 2 -concentrating mechanism (CCM) is highly regulated, responds to illumination/darkness cycles, and allows cyanobacteria to thrive under limiting C i conditions. While the transcriptional control of CCM activity is well understood, less is known about how regulatory proteins might allosterically regulate C i ransporters in response to changing conditions. Cyanobacterial sodium-dependent bicarbonate transporters (SbtAs) are inhibited by P II -like regulatory proteins (SbtBs), with the inhibitory effect being modulated by adenylnucleotides. Here, we used isothermal titration calorimetry to show that SbtB from Cyanobium sp. PCC7001 (SbtB7001) binds AMP, ADP, cAMP, and ATP with micromolar-range affinities. X-ray crystal structures of apo and nucleotide-bound SbtB7001 revealed that while AMP, ADP, and cAMP have little effect on the SbtB7001 structure, binding of ATP stabilizes the otherwise flexible T-loop, and that the flexible C-terminal C-loop adopts several distinct conformations. We also show that ATP binding affinity is increased 10-fold in the presence of Ca 2+ , and we present an X-ray crystal structure of Ca 2+ ATP:SbtB7001 that shows how this metal ion facilitates additional stabilizing interactions with the apex of the T-loop. We propose that the Ca 2+ ATP-induced conformational change observed in SbtB7001 is important for allosteric regulation of SbtA activity by SbtB and is consistent with changing adenylnucleotide levels in illumination/darkness cycles.