The study shows that the Shaker channel mutation is structurally different from human potassium channels

Potassium (K⁺) channels are small highly specialized channels in every living cell that are responsible for the extremely selective and rapid transport of K⁺ ions across cell membranes. Voltage-gated potassium (Kv) channels are potassium-specific transmembrane channels that are also sensitive to changes in the voltage within the cell membrane, where the selectivity filter selects K⁺ ions over sodium (Na⁺).

In addition to directing ion selectivity, the selective filter uses a process known as C-type inactivation, which allows the selective filter to act as an additional gate that can stop the flow of ions. C-type inactivation is a rearrangement of the selectivity filter that occurs when sustained depolarization of the cell membrane is triggered by the opening of the membrane’s activation gate.

In research led by Texas Tech University School of Medicine (TTUHSC) Luis Cowell, Ph.D., and Allen J., Ph.D. Labro from the University of Antwerp and Ghent in Belgium, the researchers investigated whether a known mutation (W434F) found in the Shaker-IR channel (inactivation removed) embodies and functionally accelerates the C-type inactivation state.

The Cuello-Labro team included D. Marian Cortes of TTUHSC and Laura Koonen, Evelyn Martinez-Morales, Dieter W. Van De Sande, and Dirk J. Snyders from the University of Antwerp. Their study, The nonconducting W434F mutant adopts an inactivated-like state upon membrane depolarization that differs from wild-type Shaker-IR potassium channels”, was published in September Achievements of science.

The unique transport mechanism of K⁺ ions determine the correct functional behavior of each living cell. At the same time, K⁺the transport mechanism effectively controls various highly complex processes, such as the normal electrical activity of brain neurons, the typical immune response of the human body against life-threatening pathogens, and the rhythmic beating of the human heart.

In the human heart, one of them potassium channels, the hERG channel, must undergo C-type inactivation before it can function as required to maintain heart rate or beat-to-beat interval. Cuella said these potassium channel proteins are dormant in a normal cell, but they need to be activated to work properly. However, for normal human heart function, it is essential that they undergo C-type inactivation.

“Once activated, the channel must be deactivated,” Cuella explained. “It is very important for the human heart because it needs to maintain its periodicity, and on this feature the heartbeat is based; the channel must be deactivated.”

To learn more about Kv channels in humans, scientists have spent years studying a specific mutation (W434F) in the Shaker channel, a potassium channel derived from Drosophilia melanogaster, a common species of fruit fly. Similar to potassium channels in the human body, the Shaker channel is composed of vital membrane proteins that play an essential role in the proper functioning of the cell and participate in the functioning of potassium ion channels.

The Shaker channel was discovered in the US about 50 years ago and became widely available, especially to those studying potassium channels, because it was the only channel available. When the mutation was found in the channel – W434F – it was widely accepted to represent an inactivated state of the C-type potassium channel. Cuella said his team showed that wasn’t the case.

“In this paper, we demonstrate that the normal, deactivating channel has a structure and conformation different from that of the mutant,” Cuella said. “With our work, we said, ‘Hey, be careful because this mutant channel has a completely different conformation, or you could have a different structure that has nothing to do with a real channel in the heart or in the human body.’ This is important because if you want to develop a drug based on the assumption that the normal channel looks like the mutant channel, it will be wrong, because it seems that in the inactive state the structure of W434F is different from our experimental result.”

The mutant channel was used because it essentially creates an ion-impermeable channel. Cuella said that there have been some experiments that show that the mutant does not conduct ions because it is inactivated like normal potassium channels in the human body. However, the results of his team’s experimental studies convincingly indicate that the mutant channel is trapped in a deeply inactivated and physiologically irrelevant conformation. And because human potassium channels closely resemble those of the mutant, the Shaker channel has for years been considered a structural surrogate for studying all channels.

“This particular mutant (W434F) doesn’t compare well to the inactivated human C-type channel that we have in our bodies, so we have to be careful with it,” Cuella stressed.

The channel was also thought to lack potassium or sodium selectivity, but Cuella said his team’s work proved that this was not the case.

“The channel is still selective for potassium; the only thing it doesn’t do is conduct potassium,” Cuella said. “This is important when you want to develop new drugs, because it gives you information about what part of the protein you should use to make it a therapeutic target. In this case, we look at our work and say, ‘Hey, don’t establish a structural parallel between the mutant inactivated channel and the inactivation state of the normal channel in Human body.’ Keep in mind that there is no structural equivalent, and this is very important.”

As research progresses, Cowell’s lab is moving away from studying the K. fruit fly⁺ and began working with the three human K. isoforms⁺ channels. Two of the channels they work with are found in the immune system and one in the brain.

“We want to study what happens when these channels are deactivated,” Cuella reasoned. “How does this process happen and how can we develop specific drugs to deal with a specific channel condition? Do we need to target the channel when it is in closed state or do we need to fight with channel when is it open? We need to start studying the human isoform of all potassium channels if we want to develop specific drugs for specific diseases.”