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A balanced potash home is essential for the survival of people and bacteria. Since bacteria are exposed to much greater fluctuations in environmental conditions, controlled potassium intake is often a particular problem. Since the cell membrane is impenetrable to potassium ions, it must be translocated via specific membrane transport proteins.
On the one hand, potassium channels allow a rapid but passive influx of potassium ions. This stops as soon as an electrochemical balance between the cell and its environment has been reached. To reach intracellular concentrations going beyond, potassium is actively transported into the cell through potassium pumps, the energy being consumed in the form of ATP.
Since the two families of proteins, channels and pumps, perform very different functions, they have always been described as distinct. This is however contradicted by the observation that KdpFABC, a highly affine and active potassium uptake system, does not represent a simple pump, but is made up of a total of four different proteins. One of them is derived from a typical pump, while another looks like a potassium channel.
Inga Hänelt, assistant professor of biochemistry at Goethe University, and her colleague Cristina Paulino from the University of Groningen, the Netherlands, decided to take a closer look at the membrane protein KdpFABC at microscope – or more precisely the cryoproton microscope. They were surprised by the result: "All the previous assumptions were wrong," says Inga Hänelt. "Although we had all the data, it took us some time to understand the path that potassium takes to move from the complex to the cell."
First, a channel-shaped protein binds potassium and transports it through the first tunnel to the pump. Once it has arrived, the first tunnel, turned to the outside, closes, while a second tunnel, turned inward, opens. This tunnel also extends between the two proteins and eventually ends inside the cell. "The complex essentially combines the best qualities of both protein families," says Charlott Stock, PhD student of Inge Hänelt's research group. "The channel-like protein binds potassium at the beginning in a very specific way and with a high affinity, while the pump allows active transport that can enrich 10,000 times the potassium in the cell."
The data, recently published in Nature Communications, impressed the scientists with the diversity of transport across the membranes. "We have learned that by investigating various membrane transport proteins, we should not rely on seemingly irrefutable mechanisms, but that we must be prepared for any surprises," Inga Hänelt summarizes.
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Material provided by Goethe University Frankfurt. Note: Content can be changed for style and length.
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