New aspects for coordination of cyanobacterial metabolism: Unravelling the regulatory network of SbtB and c-di-AMP signalling

• Applicant: Professor Dr. Khaled Selim
• Subject Area: Metabolism, Biochemistry and Genetics of Microorganisms ; Microbial Ecology and Applied Microbiology
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 545740724

Members of the PII signaling superfamily are versatile, multitasking signaling proteins found ubiquitously in all domains of life. They adeptly monitor and synchronize the cell's carbon, nitrogen, energy, redox, and diurnal balances, primarily by binding interdependently to adenyl-nucleotides, including second messengers like cAMP and c-di-AMP. These proteins also undergo a variety of posttranslational modifications such as phosphorylation, carboxylation, and disulfide bond formation, which further provide cues on the cellular metabolic states. In addition to canonical PII proteins involved in carbon/nitrogen sensing, structurally similar PII-like proteins evolved to fulfill diverse cellular functions, yet poorly understood. Recently, we identified the PII-like protein SbtB in cyanobacteria as carbon sensing module via sensing various adenine nucleotides including the second messenger nucleotides cAMP, and c-di-AMP, involved in global cellular homeostasis. We showed that cAMP acts as carbon signal, whereas adenyl-nucleotide binding links SbtB signalling to the energy state of the cells. The c-di-AMP signaling through SbtB turned out pivotal for day-night acclimation of cyanobacteria via regulation of glycogen metabolism through its interaction with the glycogen-branching enzyme GlgB. To our knowledge, this is the first signaling protein known integrating both cAMP and c-di-AMP signaling. Additionally, we showed a broad regulatory impact of c-di-AMP signaling on cellular metabolism, independent from SbtB, supporting the notion of involvement of additional, yet unknown, c-di-AMP receptors in controlling cyanobacterial metabolism. Moreover, SbtB possess a C-terminal extension with a disulfide bridge, which we call R-loop (Redox-sensitive loop). We revealed an unusual redox-dependent diphosphohydrolase activity of SbtB, that is controlled by the R-loop, to hydrolyze ATP/ADP to AMP for metabolic switch between adenyl-nucleotides to modulate the activity of the HCO3- transporter SbtA, and thereby acting as a valve plug to control the carbon flow into the cell. Serving as precise metabolic sensors, SbtB seems to convey this information to diverse cellular targets, yet enigmatic, establishing dynamic regulatory assemblies that fine-tune cellular homeostasis. Further, we identified new promising cellular targets for both SbtB and c-di-AMP through several pull-down experiments. In this research proposal, we will focus on the new cellular functions of SbtB and c-di-AMP signaling to explore the molecular mechanisms governing the dynamics of their cellular functions. Thus, we aim to decipher the molecular details of SbtB-Stomatin/Flotillin and SbtB-HisB (imidazoleglycerol-phosphate dehydratase) complexes in controlling thylakoid membranes biogenesis and histidine biosynthesis, respectively. Additionally, we aim to decode the molecular and structural details of c-di-AMP signaling in controlling the cyanobacterial natural competence and Na+-bioenergetics.

• DFG Programme: Emmy Noether Independent Junior Research Groups
• Major Instrumentation: FPLC
• Instrumentation Group: 1350 Flüssigkeits-Chromatographen (außer Aminosäureanalysatoren 317), Ionenaustauscher