Solid-state nuclear magnetic resonance spectroscopy has been used for the first time to investigate large membrane proteins in bacteria, allowing researchers to investigate exactly how the sensory systems of these single-celled organisms function.

Bacteria seem to be ubiquitous on earth. Every niche from the earthiest rock to the interior of your intestine, from volcanic vents to desert sands harbour untold billions. Their adaptability to the extremes of conditions in which they can thrive depends in part on their ability to rapidly detect changes in their surroundings and react to them.

Now, scientists at the Johannes Gutenberg University of Mainz, Germany, are figuring out how bacteria manage to assimilate readings from their environment through membranes into the cell nuclei that control their next move.

"The sixty-four-thousand-dollar question is how signals are transmitted across the cell membrane," says Gottfried Unden of the Institute of Microbiology and Virology. He is working in collaboration with colleagues at the Max Planck Institute for Biophysical Chemistry in Göttingen. Their latest results suggest that structural alterations to membrane-based sensory proteins play an important role in the transfer of signals.

Unden explains that some bacteria have more than 100 different sensors that are used to build up a complete picture of the bacterial environment. These sensors monitor nutrient levels, the surrounding oxygen concentration, keep tabs on temperature and light levels, and more. The majority of these bacterial biosensors, perhaps obviously, is to be found in the cell membrane that separates the bacterium from its surroundings.

Using the latest tools from molecular biology, the researchers have isolated these sensors and applying NMR to investigate their function and form. The Mainz team first modified one sensor protein that is involved in the detection of a bacterial substrate to make it amenable to solid-state NMR. "This is the first time that solid-body nuclear magnetic resonance (NMR) spectroscopy has been used to investigate large membrane proteins," says Unden.

The team of biophysicists at Göttingen headed by Marc Baldus carried out parallel work to identify the details of the chemical communication process. They have identified carbonic acid as a stimulus molecule. It binds to a part of the sensor that protrudes from the cell, which then triggers dissolution of the ordered structure of that segment of the sensor within the cell. This new plasticity then activates a cascade of bacterial enzyme reactions ultimately triggering a cellular response, to generate the enzymes required to deal with the stimulus or protect the bacterium.

The microbiologists at Mainz University have also discovered what turns out to be a rather extraordinary property of signal detection in the same protein sensor, DcuS. They discuss details in Journal of Biological Chemistry. This work reveals that bacteria react not only to the outside environment but also respond to intracellular stimuli. This suggests that the sensors are not alone in detecting stimuli.

The researchers suggest that a second stimulus detection pathway is involved that coincides with the cellular transport system that carries nutrients and other substances into the cell. Unden says that once carbonic acid has been absorbed, the transporter system then notifies the sensor of this action.

"We have been able to identify that segment of the transporter that is responsible for the control of sensor functioning," Unden explains, "The transporter is of fundamental importance for the function of the sensor. Without the transporter, the sensor does not work correctly and is constantly in an activated state." Underpinning this phenomena is the presumed need for a feedback loop that controls metabolic and transport activity, which is more important in some ways than information concerns.

The team worked with the DcuB sensor of Escherichia coli, which catalyses the C4-dicarboxylate/succinate process while the bacteria grows and respires fumarate. They demonstrated that the genes for proteins involved in fumarate respiration, include DcuB (dcuB) and the gene for fumarate reductase. These are activated by C4-dicarboxylates through the DcuS-DcuR system. DcuB is a bifunctional protein and has a regulatory function that is independent of transport systems and so sites for transport and regulation can be differentiated, the team explains.

Related links:

• J Biol Chem, 2009, 284, 265-275
• Nature Struct Mol Biol, 2008, 15, 1031-1039 
• Unden Dept (German)

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.