"It is puzzling that the human genome has only slightly more genes than the fruit fly and only half the number of cauliflower. Obviously, the sheer number of genes alone cannot determine the complexity of human development and the sophisticated signalling systems that maintain homeostasis. In fact, the complexity of information from genome to proteome is greatly increased. Drivers for the complexity of the proteome are posttranslational modifications (PTMs), i.e. covalent modifications of polypeptides after translation. <br/>The aim of this project is to shine light on a scientific mystery known for decades. We will elucidate the cellular roles and functions of diadenosine polyphosphates (ApnAs), which are formed in response to stress and are therefore called ""alarmones"", and their interactions with the PTM processes ""AMPylation"", i.e. the covalent modification of the target protein by adenosine monophosphate. Although both topics are known for more than 60 years, their molecular mechanism and functions are poorly understood. <br/>Based on our preliminary results, which unambiguously show the interplay of ApnAs and AMPylation for the first time, we will search for, identify and characterize new protagonists of AMPylation and clarify their interactions with ApnAs. Therefore, new chemical tools will be developed and applied in the proteome-wide studies in living cells.<br/>We will also investigate the molecular function of a protein that we have already identified with ApnA-based probes. We have discovered that this protein is a ""5'-3' RNA ligase"", the first one found in human cells! The available data indicates the involvement of this RNA ligase in cancer and neurodegeneration, and we will therefore elucidate the molecular mechanisms and functions of this protein in detail.<br/>Overall, this project will bring significant new and previously unexplored advances and will provide a guideline for future translational research in the fight against diseases."