Structural and Biophysical Characterization of the Cytoplasmic Domains of HprS Kinase and Its Interactions with the Cognate Regulator HprR

The HprSR constitutes the bacterial two-component regulatory system engaged by Escherichia coli to reduce the damaging effects of reactive chlorine and oxygen species present in its cytosol. Hypochlorous acid (HOCl) has been shown to be the molecule capable of activating of the HprSR system. HOCl is produced upon pathogen invasion by phagocytic cells of the human innate immune system, particularly neutrophils, to take advantage of its powerful antimicrobial attributes. Therefore, comprehensive studies concerning bacterial sensing and regulatory HprSR system are indispensable in understanding and effectively eliminating pathogens. Here we present the first crystal structure, solved at 1.7 Å resolution, of the HprS cytoplasmic domains arranged as a homodimer. In both protomers, the catalytic ATP-binding domain contains a non-hydrolysable ATP analog coordinated by a magnesium ion. This structure allowed us to provide a detailed characterization of kinase-substrate interaction. Furthermore, the structural data are supported by biophysical studies of kinase interaction with cognate response regulator HprR and substrate ATP. The kinase activity is also assessed in the presence or absence of HprR.