Regulation of Porins in Escherichia coli by the Osmosensing Histidine Kinase/Phosphatase EnvZ

EnvZ, the osmotic sensor for Escherichia coli, belongs to the largest class of histidine kinases. In response to changes in medium osmolarity, EnvZ regulates the level of phosphorylated OmpR (OmpR-P), its cognate response-regulating transcription factor for ompF and ompC genes. EnvZ has dual-opposing enzymatic activities: OmpR-phosphorylase (kinase) and phospho-OmpR-dephosphorylase (phosphatase). The osmotic signal is proposed to regulate the ratio of the kinase to the phosphatase activity of EnvZ to modulate the level of OmpR phosphorylation. The C-terminal kinase domain of EnvZ has been dissected successfully into two independent domains: A and B. This chapter describes results that shed new light on various aspects of EnvZ function. It demonstrates that domain A is not simply a structural scaffold for the EnvZ dimer formation by forming a central four-helix bundle, it also plays an essential role in the phosphatase reaction, providing the invariant histidine residue at the active center. Domain A by itself is able to dephosphorylate OmpR-P, and phosphorylated domain A is able to transfer the phosphoryl group to OmpR. However, domain B plays an essential role in phosphorylation of the invariant histidine residue in domain A with ATP and significantly enhances domain A phosphatase activity when it is linked covalently to domain A. The chapter also discusses how the opposing enzymatic activities of EnvZ are regulated on the basis of the biochemical characterization of individual domains A and B, mutagenesis analysis of these domains, and the experiments using Tazl, a Tar.