Enteric bacteria such as Escherichia coli, Salmonella typhimirium and Vibrio cholera possess a wide array of acid stress response systems to counteract the extreme acidity encountered when invading the host’s digestive or urinary tracts. This project aims to analyse structure-functional relationships of the inducible lysine decarboxylase LdcI system to deeper understand its mechanism of action and its role in acid resistance, and to explore its newly described link with the stringent response. Indeed, we recently discovered that LdcI tightly binds to the procaryotic stringent response regulator alarmone. In addition, we identified LdcI as interacting with a novel AAA+ ATPase RavA that appears to have been specifically evolved to target LdcI. Moreover, we found that RavA reduces the inhibition of LdcI activity by the alarmone in vitro as well as in vivo. We solved Xray crystal structures of the LdcI decamer and of the RavA monomer, and obtained three-dimensional reconstructions of LdcI, the RavA-ADP hexamer and the LdcI-RavA-ADP complex by electron microscopy. The latter complex revealed itself as a suprising cage structure composed of two LdcI decamers linked together by five RavA hexamers. A combination of the Xray crystal structures with the low resolution electron microscopy maps enabled us to propose an atomic model of the RavA hexamer and get first insights into the basis of its interaction with LdcI. Furthermore, we have evidence for a ternary complex formation between LdcI, RavA and a novel metal-binding VWA domain protein ViaA. Our consortium will continue to explore the interplay between LdcI, RavA, ViaA, nucleotides as an energy source for RavA and alarmone as a stress signal molecule, by an intergated multi-level approach envolving structural, molecular and in vivo cell biology. In the scope of this grant application, we will focus on providing structural insights into this crucial enteric bacterial acid stress response system by high resolution cryoelectron microscopy. Unraveling these interactions is of particular interest for two reasons. First, this knowledge should be profitable for future clinical use, since bacterial infectivity correlates with their ability to withstand acid stress. Thus, blocking or regulating the LdcI interaction with its partners is a potential target for pharmaceutical intervention. Second, understanding the interaction in such an intriguing supramolecular complex as LdcI-RavA will greatly add to our knowledge of the basic principles of specific protein-protein recognition, which play central roles in all biological processes.