Mechanism of bacterial signal transduction deciphered

The investigated sensor histidine kinase can switch between a kinase and a phosphatase form in response to blue light. In doing so, it either transfers phosphate groups to other proteins or removes them. The stylized protein structures show spatial arrangement of the protein in the two states. Protein crystals used for the structural analyses are shown in the background. © Ulrich Krauss/Renu Batra-Safferling/ChatGPT Image 2

SHKs are part of two-component systems in bacterial signalling pathways. They consist of a sensor that detects environmental stimuli such as light, and a regulator that controls the response to a stimulus such as altered gene expression. Depending on the system, SHKs can trigger different responses in light or darkness: they either transfer phosphate groups to a regulator or remove them. This regulator then controls further molecular processes within the cell, such as changes in gene activity. Although SHKs are responsible for a wide variety of signalling pathways in bacteria and are also used in optogenetics – a biological technology that controls cellular activity with light – the precise mechanisms of signal transmission remain largely unclear. This problem was addressed by a research team from the University of Bayreuth and Forschungszentrum Jülich.

“In our study, we examined newly engineered light-sensitive SHKs, which can be used as optogenetic tools, both structurally and functionally. We combined crystallographic data with analyses of the proteins in solution and functional tests,” says Professor Dr. Ulrich Krauss from the Biochemistry I research group at the University of Bayreuth, who is also a visiting scientist at the Institute of Molecular Enzyme Technology of Heinrich-Heine-University Düsseldorf, which is part of Forschungszentrum Jülich’s Institute of Bio- and Geosciences IBG-1. This combination of methods enabled the researchers to accurately assign the active dark state and the light-induced structural change of the SHKs. “Close collaboration between different institutes and research groups at Forschungszentrum Jülich and the University of Bayreuth was crucial for deciphering the mechanism of signal transduction,” adds Professor Dr. Renu Batra-Safferling, head of the Protein Biochemistry research group at Forschungszentrum Jülich.

In the SHKs studied, an asymmetrically bent form occurs in the dark, whereas light favours a symmetrical, straight structure. “The transition between these two forms is closely linked to the kinase activity of the enzyme, i.e. the transfer of phosphate groups to the regulator,” explains Batra-Safferling. Functional and structural investigations of the enzyme in solution suggest that the asymmetrically bent form possesses kinase activity, whereas the symmetrical, straight form exhibits phosphatase activity, i.e. it removes phosphate groups from the regulator. Thus, the spatial structure of the SHK largely determines whether phosphate groups are transferred or removed – a central step in the regulation of gene activity by two-component systems. “Put simply, in the asymmetrical form, key parts of the enzyme are oriented in such a way that kinase activity is possible. The transition to the symmetrical, straight form changes this arrangement and renders the components incompatible with the kinase function. This favours phosphatase activity over kinase activity,” Krauss adds.

The researchers are currently investigating how universal this mechanism is for other SHKs and what role protein dynamics generally play in signal transduction in light-sensitive proteins in a further project.

The study was conducted as part of a project funded by the German Federal Ministry of Education and Research (BMBF) (OptoSys; funding codes 031A16 and 031A167B) and was also supported by the German Research Foundation (DFG).