PhoP-PhoQ

Introduction

PhoP-PhoQ is a two component regulatory system found in some gram-negative bacteria such as Escherichia coli[1], Salmonella enterica Typhi[2], and Yersinia pestis[3]. In a classic two component regulatory system, there exists a sensor kinase and a response regulator^[1]. In the phoP-phoQ system, phoQ acts as the sensor kinase and phoP acts as the response regulator. The purpose of this signal transduction system in bacteria is to modify cellular output in response to environmental signals. In response to particular environmental stimuli, such as a low [Mg²⁺], the sensor kinase, phoQ autophosphorylates. Phosphorylated phoQ then transphosphorylates the response regulator, phoP, which in turn binds DNA and modulates transcription.^[1] For more details see PhoP Regulatory Domain.

PhoP: The Response Regulator

E. coli phoP is a 223 residue protein containing a 120-residue regulatory domain joined by a 5-residue linker to a 98 residue c-terminal DNA binding effector domain.^[2] At a conserved Asp residue (), the regulatory domain can be modified by a phosphoryl group from the protein kinase function of phoQ. Phosphorylation of the regulatory domain modulates the activity of the effector domain to bind DNA and regulate transcription. Phosphorylated, or "activated" phoP binds to "phoP boxes" on bacterial DNA, which consist of two direct hexanucleotide repeats separated by a five nucleotide spacer located at the -35 position.^[2]: (T/G)GTTTA

Dimerization of Activated phoP

The activated phoP regulatory domain dimerizes with two-fold symmetry using the face formed by ^[2]. The phosphoryl group donated from phoQ is used to stabilize the active conformation by inducing dimerization. Due to the absence of an intramolecular domain interface which precludes direct transmission of an activation signal, this function as a dimerization motif seems to be the phosphoryl group's only role.^[2]

Beryllofluoride Coordination

In this structure, (BeF₃^-) is used as a phosphoryl analog to induce the active state conformation. There are numerous bonds that form as a result of BeF₃^- coordination. F₁ of BeF₃^- helps satisfy the octahedral coordination of a present active site ²⁺. The conserved active site lysine, , forms important intramolecular and intermolecular salt bridges, one of which is with F₃ of BeF₃^-. A conserved "switch residue" , involved in the activation of all response regulators, forms a H-bond with F₂ of BeF₃^-. F₂ of BeF₃^- also forms a H-bond with the backbone nitrogen atom of .^[2]

Spontaneous Dimerization

Spontaneous dimerization of the unactivated phoP regulatory domain can be observed in vitro, but is likely due to high concentration of the protein and may not occur in vivo. Although the dimerization of the unactivated phoP regulatory domain results in a homodimer similar to the activated homodimer, it is significantly less stable. Activation by the phosphoryl group helps to stabilize the dimerization interphase. Without the phosphoryl group, the two monomers dimerize in an asymmetric fashion that lends to molecular instability.