Regulatory adenylnucleotide-mediated binding of the PII-like protein SbtB to the cyanobacterial bicarbonate transporter SbtA is controlled by the cellular energy state
Cyanobacteria have evolved a remarkably powerful CO2 concentrating mechanism (CCM), enabling high photosynthetic rates in environments with limited inorganic carbon (Ci). Therefore, this CCM is a promising system for integration into higher plant chloroplasts to boost photosynthetic efficiency and yield. The CCM depends on active Ci uptake, facilitated by bicarbonate transporters and CO2 pumps, to elevate CO2 concentration around the active sites of the primary CO2 fixing enzyme, Rubisco, which is encapsulated in cytoplasmic micro-compartments (carboxysomes). The essential CCM proteins have been identified, but the molecular signals and regulators that coordinate function in response to light, Ci availability and other environmental cues are largely unknown. Here, we provide evidence, based on a novel in vitro binding system, for a role of the PII-like SbtB protein in regulating Ci uptake by the bicarbonate transporter, SbtA, in response to the cellular adenylate energy charge (AEC) through dynamic protein-protein interaction. Binding of the SbtA and SbtB proteins from two phylogenetically distant species, Cyanobium sp. PCC7001 and Synechococcus elongatus PCC7942, was inhibited by high ATP, and promoted by low [ATP]:[ADP or AMP] ratios in vitro, consistent with a sensory response to the AEC mediated through adenylnucleotide ligand-specific conformation changes in SbtB. In vivo, cell cultures of S. elongatus showed up to 70% SbtB-dependent down-regulation of SbtA bicarbonate uptake activity specifically in the light activation phase during transitions from dark to low light when low cellular AEC is expected to limit metabolic activity. This suggests SbtB may function as a curfew protein during prolonged low cellular AEC and photosynthetically unfavourable conditions to prevent energetically futile and physiologically disadvantageous activation of SbtA.