Cytoplasmic sensing by the inner membrane histidine kinase EnvZ

Area of Science:

• Bacterial physiology
• Molecular biology
• Signal transduction

Background:

• Two-component systems regulate bacterial signal transduction.
• EnvZ and OmpR control outer membrane protein expression in response to osmotic stress.

Purpose of the Study:

• Identify conformational changes in EnvZ's cytoplasmic domain during osmosensing.
• Investigate the link between osmolality, EnvZ autophosphorylation, and gene regulation.

Main Methods:

• Hydrogen:deuterium exchange mass spectrometry (HDX-MS) to detect protein conformational changes.
• Site-directed mutagenesis and reporter gene assays to assess EnvZ/OmpR system activity.

Main Results:

• Osmosensing localized to a 17-amino acid region within EnvZ's four-helix bundle, near the autophosphorylation site.
• High osmolality increased EnvZ autophosphorylation, suggesting a direct link.
• Transmembrane domains were not essential for sensing but influenced activity; EnvZ/OmpR system lacks robustness.

Conclusions:

• Osmolytes stabilize EnvZ helix one via hydrogen bonding, enhancing autophosphorylation and downstream signaling.
• A model for osmosensing involving direct stabilization of the EnvZ cytoplasmic domain is proposed.
• Similar sensing mechanisms may exist in other histidine kinases.