The KR2¹ protein transports positively charged sodium ions outside of the cell, a feature that optogenetics researchers have been unable to harness. Until now, the KR2 atomic structure and mechanism of action were unknown. Researchers at the IBS, the Moscow Institute of Physics and Technology (Russia) and the Institute of Complex Systems (Forschungszentrum Jülich, Germany) took up this challenge with X-ray crystallography techniques. They thus obtained an exact 3D reconstruction of the KR2 atomic structure, as well as the structure of the molecular complex that it forms under physiological conditions.

The structure of KR2 has novel features, including a helical portion covering the extracellular pump entrance, like a lid. One feature of particular interest to researchers is the unusual structure of its ion loading cavity on the intracellular side, which is uncharacteristically wide and protrudes into the cytoplasm, relative to the membrane surface. This structure could act as a filter lying behind KR2's selectivity for sodium ions.

To test this hypothesis, the researchers modified the structure of this "filter" by changing the specific amino acids of its molecular site with targeted mutations. Not only did KR2 effectively lose its competence regarding sodium ions, but one of these mutations transforms KR2 into a photosensitive potassium pump – a first of its kind.

These results are particularly interesting due to their potential applications in optogenetics. An activated neuron normally returns to a state of rest by releasing potassium ions through membrane ion channels. A mutated KR2 potassium pump could be utilized to "turn off" an active neuron at will, using light pulses. This would therefore constitute a specific control system for use by researchers.

The authors of the study are now working to develop a method to integrate this system into different types of neurons by transfection², and to improve the targeting of these proteins in the plasma membrane of transfected neurons.

 

1. In 2013, scientists working on the marine bacterium Krokinobacter eikastus discovered a protein in the membrane serving as an ion transporter of an unknown type. Dubbed KR2, it belongs to a group of proteins that have become the pillars of "optogenetics" research. When exposed to light, this protein family can modify the electrical potential of cells by transporting charged particles across the membrane.

2. A process of transferring genes, i.e. the direct introduction of genetic material into cells.