Structural changes in bacterial osmosensing

Conformational changes in the histidine kinase EnvZ facilitate bacterial osmosensing.

Prof Linda Kenney, principal investigator at MBI and Professor of microbiology at the University of Illinois-Chicago, probes the mechanics of ‘two-component systems’ in bacterial signal transduction. ‘Two-component systems’ are based on simple stimulus-response scenarios and are fundamental to bacterial survival. Typically the sensor half of the system comprises a membrane-bound histidine kinase (HK), whilst the response half of the system consists of a cytoplasmic response regulator (RR) that usually affects gene expression. This set-up allows bacteria to sense and respond to a wealth of different environmental conditions. Understanding how bacteria function and survive will improve methods of targeting and eliminating those that are detrimental to human health.

Stretch-relaxation model for EnvZ osmosensing. Low osmolality favors a stretched conformation (left) and high osmolality favors more stable helices (right). Image courtesy of Linda Kenney and featured in The EMBO Journal (PMID: 22543870).

The model at last provides a physical basis for how EnvZ can respond to diverse chemical osmolytes such as sucrose and NaCl, says Prof Linda Kenney.

The Research

The model system employed in the Kenney lab is that of Salmonella enterica, a rod-shaped bacterium that when ingested causes salmonellosis – a condition marked by gastrointestinal symptoms such as diarrhoea, vomiting and abdominal cramps. S.enterica employs the ‘two-component system’ of EnvZ/OmpR. The sensor EnvZ, a histidine kinase present in the inner leaflet of the plasma membrane, detects external changes in osmolality. The response regulator OmpR, a DNA binding protein, activates genes required for systemic infection. The precise mechanism of how these two proteins work together to transduce a change in the extracellular medium to a change in gene expression, is not entirely clear. Recent work in the Kenney lab however sheds light on the conformational changes that occur in EnvZ to facilitate this process.

Molecular pathway of EnvZ/OmpR osmosensing. Low osmolality (left): the major porin in the outer membrane is OmpF and levels of OmpR are low. High osmolality (right): OmpR levels increase, ompF expression decreases, ompC expression increases and OmpC becomes the major porin. Image courtesy of Linda Kenney.

Recent Findings

Results from the Kenney lab published in The EMBO Journal entitled ‘The inner membrane histidine kinase EnvZ senses osmolality via helix-coil transitions in the cytoplasm’, highlight the molecular basis for the mechanotransduction of osmolality in Salmonella enterica. Using specialized mass spectrometry techniques (hydrogen deuterium exchange mass spectrometry (HDXMS)), Kenney and colleagues demonstrated that increased osmolality increased the helical nature of the cytoplasmic domain of EnvZ (EnvZC) and also increased its autophosphorylation at histidine-243. A model was therefore proposed where an increase in osmolality enhances helix stability of EnvZC, which promotes its autophosphorylation and leads to downstream signaling to OmpR.

Interestingly, the EnvZ cytoplasmic domain alone was shown to be sufficient for osmosensing, whilst transmembrane domains were dispensable. These novel findings raise more questions, such as why EnvZ is localized to the membrane if membrane localization is not required for signal transduction?

Work in the Kenney lab continues to search for the answers. Learn more.

 

Article by Lux Fatimathas