A new study by researchers from the University of Illinois measures movements smaller than one-billionth of a meter in ion channels. This movement is critical to how these tiny pores in the cell membrane open and close in response to changes in voltage across the membrane. The findings appear this week in the journal Neuron.
Ion channels belong to a special class of proteins embedded in the oily membranes of the cell. They regulate the movement of charged particles, called ions, into and out of the cell. Much like water faucets that can be controlled by turning a knob, channels open or close in response to specific signals. For instance, ion channels that open in response to pressure on the skin regulate our sense of touch.
Voltage is an important switch that controls how some channels open. The voltage across the cell membrane depends on the balance of ions inside and outside the cell and also on the type of ions. Voltage-gated channels are critical for transmitting messages from the brain to different parts of the body by means of nerve cells.
“There has been a large controversy in the field with regards to how these channels respond to voltage,” said University of Illinois physics professor Paul Selvin, who led the study. The controversy centers on a key segment of the ion channel called the voltage sensor.
The voltage sensor gauges the voltage across the membrane and instructs the channel to open or close.
One model for the movement of the voltage sensor suggests that it moves up and down by only a small amount, tugging on the pore of the ion channel and opening it just enough for ions to get through. In 2003, Roderick MacKinnon, who won a Nobel Prize in chemistry for his work on the X-ray crystal structures of ion channels, proposed a competing idea, the “paddle model.” This idea involved a large movement of the voltage sensor across the membrane. X-ray crystal structures provide snapshots of proteins in exquisite detail, allowing researchers to look at the positions of every atom.
According to Selvin, a problem with the crystal structure is that it only offers a static snapshot of what the protein looks like and provides only limited information about how different parts of the protein move. Another concern is that the conditions used to obtain protein crystals sometimes alter the original structure of the protein.
In the new study, postdoctoral researcher David Posson worked with Selvin to put the models of voltage sensor movement to the test.
They studied the voltage sensor segment in a specific ion channel called the Shaker potassium channel. This protein was first discovered in fruit flies after researchers observed that a mutation in the channel caused the flies to vigorously shake.
To preserve channels in their original state, Posson studied ion channels inserted into the membranes of frog eggs. He tested the two models using a fluorescence technique called Lanthanide resonance energy transfer (LRET) which allowed him to measure small movements in proteins. The technique involves the use of a special pair of molecular bulbs that glow either brightly or dimly depending on how far apart they are. The measurement is sensitive to movements as small as one-billionth of a meter. Posson also needed a way to control the voltage across the membrane.
He used an approach called electrophysiology that involves inserting electrodes into the frog egg. This gave him the ability to change the voltage across the membrane and regulate channel opening.
“Our approach brings together two distinct biophysical techniques, electrophysiology and fluorescence, which have been independently useful for the study of proteins,” Posson said.
To map the movement of the voltage sensor during channel opening, Posson measured distances from several different vantage points on the protein.“It’s a lot like dispatching a team of molecular surveyors that stand at specific positions on the surface of a protein and collect distances from point
A to point B,” Posson said. “With enough measurements, the surveyors can build a map of the three dimensional shape of the protein.” Posson discovered that the largest distances traversed by the sensor were about two to three times smaller than what was predicted by the paddle model. It showed that the sensor moves by only a small amount to allow the flow of ions.
“We are seeing a clear result that the movement of the sensor isn’t super teeny, and isn’t super huge,” Posson said. The measurements challenge models that predicted large movements of the protein segments, such as the paddle model. The findings also refute models that have a near zero movement of the sensor region. “It’s a small piece to the puzzle of how the voltage sensor moves” Selvin said.
Kaushik Ragunathan | University of Illinois
Graphene gives a tremendous boost to future terahertz cameras
16.04.2019 | ICFO-The Institute of Photonic Sciences
Mount Kilimanjaro: Ecosystems in Global Change
28.03.2019 | Julius-Maximilians-Universität Würzburg
A stellar flare 10 times more powerful than anything seen on our sun has burst from an ultracool star almost the same size as Jupiter
A localization phenomenon boosts the accuracy of solving quantum many-body problems with quantum computers which are otherwise challenging for conventional computers. This brings such digital quantum simulation within reach on quantum devices available today.
Quantum computers promise to solve certain computational problems exponentially faster than any classical machine. “A particularly promising application is the...
The technology could revolutionize how information travels through data centers and artificial intelligence networks
Engineers at the University of California, Berkeley have built a new photonic switch that can control the direction of light passing through optical fibers...
Physicists observe how electron-hole pairs drift apart at ultrafast speed, but still remain strongly bound.
Modern electronics relies on ultrafast charge motion on ever shorter length scales. Physicists from Regensburg and Gothenburg have now succeeded in resolving a...
Engineers create novel optical devices, including a moth eye-inspired omnidirectional microwave antenna
A team of engineers at Tufts University has developed a series of 3D printed metamaterials with unique microwave or optical properties that go beyond what is...
17.04.2019 | Event News
15.04.2019 | Event News
09.04.2019 | Event News
18.04.2019 | Life Sciences
18.04.2019 | Physics and Astronomy
18.04.2019 | Life Sciences