PII signal transduction superfamily acts as a valve plug to control bicarbonate and ammonia homeostasis among different bacterial phyla

Life on Earth relies on carbon and nitrogen assimilation by RubisCO and GS-GOGAT enzymes, respectively, whose activities depend on a constant supply of inorganic carbon (Ci) and nitrogen (N). Members of the PII signal transduction superfamily are among the most ancient and widespread cell signaling proteins in nature. One of their most highly conserved functions is controlling Ci- and N-transporters, a feature found in different phyla of the Archaea and in both Gram-positive and Gram-negative bacteria. Recently, we identified the PII-like protein SbtB as Ci-sensing module, mainly controlling the HCO3- transporter SbtA in cyanobacteria. Similar to canonical PII proteins, SbtB is able to bind the adenine nucleotides ATP and ADP. Unlike those, it also binds AMP and preferentially binds the second messenger cAMP and c-di-AMP. The functional significance of the binding of different adenyl-nucleotides to SbtB has remained elusive, particularly in the context of the interaction of SbtB with SbtA. By a combination of structural, biochemical and physiological analysis, we revealed that by binding to SbtA, SbtB acts as unidirectional valve, preventing the reverse transport of intracellular enriched bicarbonate. This mechanistic principle holds true for the PII protein from Bacillus acting on the ammonium transporter AmtB, suggesting an evolutionary conserved role for PII superfamily proteins in controlling unidirectional flow of different transporters.