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Abstract:
The PII superfamily consists of widespread signal transduction proteins found in all domains of life. In addition to canonical PII proteins involved in C/N sensing, structurally similar PII-like proteins evolved to fulfill diverse, yet poorly understood cellular functions. In cyanobacteria, the bicarbonate transporter SbtA is expressed with the conserved PII-like protein, SbtB, to augment intracellular Ci levels for efficient CO2-fixation. We identified SbtB as a sensor of various adenine nucleotides including the second messenger nucleotides cAMP, known as carbon-status indicator, and c-di-AMP, involved in global cellular homeostasis. Moreover, many SbtB proteins possess a C-terminal extension with a disulfide bridge. We previously implied a redox-regulatory function of this extension, which we now call R-loop. Here, we reveal an unusual ATP/ADP apyrase (diphosphohydrolase) activity of SbtB that is controlled by the R-loop. We followed the sequence of the hydrolysis reactions from ATP over ADP to AMP in crystallographic snapshots and reveal the structural mechanism by which changes of the R-loop redox state modulate apyrase activity. We further gathered evidence that this re-dox state is controlled by thioredoxin TrxA, suggesting that it is generally linked to cellular me-tabolism. Finally, we present a refined model of how SbtB regulates SbtA activity, in which both the apyrase activity and its redox regulation play a central role. This highlights SbtB as a central switch-point in cyanobacterial cell physiology, integrating not only signals from the energy state (adenyl-nucleotide binding) and the carbon supply via cAMP binding, but also from the day/night status reported by the C-terminal redox-switch.
