Biology 202
2002 First Paper
On Serendip

The axons of our neurons are the pathway for the communication that exists in our nervous system. This communication takes the form of an electric signal, also called an action potential. The action potential occurs due to a change in voltage across the membrane of the axon. The change in voltage is achieved by a change in the concentrations of the ions, Na+, Ca+, and K+(1) . The cell starts with a large concentration of potassium ions, K+, inside the cell, and a large concentration of sodium ions, Na+, outside the cell. The action potential propagates down the axon due to openings and closing of different channels allowing changing of the concentrations of the ions(10).

Channels are proteins that span the membrane of the axon. These proteins have a structure so that they can be allow ions to flow through pores that are only open at the appropriate times. Some of the channels are opened and closed by other chemicals, while some are initiated by a change in the membrane potential.

This particular K+ channel, which is greatly studied, is a voltage-gated channel. This means that the channel opens in response to a certain voltage difference that occurs across the membrane. The channel is closed when the cell is at rest. Following inactivation the channel opens via a complicated mechanism, which scientists are still trying to decipher(7)(8). The specific voltage is that which occurs after the Na+ channel has opened and allowed a significant amount of Na+ to be released from the cell. So, the K+ channel is induced to an open state by a depolarization of the membrane potential. The K+ channel opens at the beginning of the repolarization, or after the depolarization has almost reached its peak. The opening of this channel allows K+ ions to flow outside of the membrane of the cell, bringing the voltage of the cell back down to its normal level. The K+ returns to the inside of the cell through a pump that exchanges it for Na+ so that there is little voltage change.

Scientists have struggled for a long time to understand how voltage-gated channels work. "How and where changes in the structure of the voltage-sensing domains [work] to gate ion conduction is not understood," said Li-Smerin et al. in a paper published in February 2000(2). Since then research in the field of voltage-gated channels has reached great heights. Now, scientists view K+ channels as those that are best understood(3). There have been multiple experiments done to determine the structure of the channel through x-ray crystallography, flourometry, and mutagenesis(2)(4). That picture has been almost perfected. Mutations have been done to determine structures that have a big influence on the voltage sensing and gating properties of the channel. For example, Li-Smerin et al. did a series of 37 mutations that led to two groups that they call the major impact and the minor impact residues(2). They were defined as being major or minor depending on how great of an affect the mutation had on the functionality of the resulting channel. When the major residues were mapped on the structure the scientists could make references as to what kind of contribution the original residue had.

The structure of the channel consists of four subunits, each with six helical segments, referred to as S1-S6. The six helices are separated based on whether they belong to the "domain that senses the voltage or the domain that surrounds the pore"(5). S1 through S4 are the helices that are thought to be the ones that respond to the voltage change. The two remaining sections of the protein are those that make up the pore, which opens and closes to allow ions in or out. The structure of these last two are almost exactly the same as a K+ channel that is non-voltage-dependant. This is further evidence that the first four are those that interpret the voltage change, since they are the difference between the two types of channels.

Knowing the structure was the one of the first steps scientists took towards understanding the channel. The basic idea is that the S1-S4 lie on the outer side of the pore, and influence the pore defining helices, S5 and S6. Near the center of the membrane is a water-filled cavity where some drugs, such as TEA, bind to block activation(3). When the pore closes it does it via changing into a teepee like structure. See (10) for a picture of the protein structure.

A portion of S4 is positively charged, and therefore the membrane potential change moves this portion, possibly even to the other side of the membrane. The closed state of the channel is more stable than the open state due to a series of hydrogen bonds in the protein structure(8). This is why such a strong energy changes, such as the depolarization are necessary to change the structure of the channel. The exact movement of the region is unknown. Originally the debate centered on the region moving laterally, but now there is a lot of evidence showing that the helix actually rotates around its inner axis, and slightly out of the membrane at an angle. This motion is comparable to that of a corkscrew. The S4 is also in very close proximity to the pore-forming segments, S5 and S6. S4 connects directly to the S5. This is still being evaluated, but the current proposal is that the torque in the S4 is translated directly to S5 and indirectly to S6. This seems very probable because the gating of channels that aren't voltage-gated rely on a twisting in S6 to open the channel. The other possibilities are that the S4 rotates the others the way gears work, or that it moves outward from the protein complex, opening the channel(5).

There are many differences that are recognizable between voltage-gated and non-voltage-gated channels. These differences help understand how the voltage-gated channel works. We have already discussed the fact that there are additional helices on the voltage-gated channels, and that there are minute differences in the structure of the pore-defining helices. Another important structure in a channel is the "residues" that line the helices. Residues are the amino acids that reside on the transmembrane surface(2). In non-voltage gated channels that residues only interact with the lipids of the membrane that surrounds them. In voltage-gated channel these residues also interact with the additional helices(2). This is beneficial because these residues can possibly show where the interactions are between the different helices. Mutants of the residues have been studied to see which ones change the voltage-sensing domain(2). Multiple scientists are currently working on studying these mutants. Some have no or little affect, while some eliminate gating altogether. Eventually these mutants should be able to tell us exactly how the channel is working.

Even without the difficulty of voltage-dependency scientists still don't know what the basic structure of the opening/closing structure of any of the channels are. It was originally thought that the channel was just a trap-door apparatus. Spin-labeling has shown that at least non-voltage-dependant channels work by rotating the inner helix(5). This may also be the formation in the voltage-dependant channel. There are two other types of gating observed in voltage-gated channels; N-type, and C-type. N-type is also referred to as the activation gate. It is near the interior of the cell, and it is open during activation. The C-type is also referred to as the slow inactivation gate, and it is located at the external part of the channel(7). Scientists are still working on identifying exactly where these sites occur. A major way of studying them is by blocking them with molecules of similar size to K+.

Eventually there will be a lot of helpful medical advances coming out of the knowledge of this channel works. Many diseases, from depression to Alzheimer's could possibly be affected by further knowledge of the ion channels. Drugs can be made that interact in more precise ways with the channels, maybe with less negative side affects. The possibilities are endless, and the knowledge is fast approaching.

References

1)Serendip Notes, Some notes are available from the class on the Neurobiology of Behavior which discuss Action Potentials.

2)A Localized Interaction Surface for Voltage-Sensing Domains on the Pore Domain of a K+ Channel

3)Potassium Channel Mechanics

4)Reconstructing Voltage Sensor-Pore Interaction from a Fluorescence Scan of a Voltage-Gated K+ Channel

5)Taking Apart the Gating of Voltage-Gated K+ Channels

6)Reconstructing Voltage Sensor-Pore Interaction from a Fluorescence Scan of a Voltage-Gated K+ Channel

7)Tight Steric Closure at the Intracellular Activation Gate of a Voltage-Gated K+ Channel.

8)The Polar T1 Interface Is Linked to Conformational. Changes that Open the Voltage-Gated Potassium Channel

9)Visual of the K+ Channel

10)Lights, Camera, Action Potential, A site made for children that has great descriptions and visuals of an action potential.

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