London, April 8 : University of Illinois researchers say have mapped the interior of a key component of the relay system that allows the neurotransmitter acetylcholine to carry impulses from neurons to skeletal muscle cells and many parts of the nervous system.
The researchers say that their study reveals how the muscle nicotinic acetylcholine receptor-a neurotransmitter-gated ion channel that regulates the flow of information in the form of charged particles (ions) across the cell membrane-responds to a burst of acetylcholine on the surface of a cell.
When the ion interacts with acetylcholine or nicotine on the surface of the cell, it causes the gate to open, allowing positively charged ions (cations) to flow into the cell.
The Illinois researchers, who claim to be the first to infer the structure of the open channel conformation in a living cell, revealed that they were able to do it by exploiting electrical properties of the membrane proteins.
Professor Claudio Grosman and a postdoctoral associate in his lab, Gisela D. Cymes, involved the patch-clamp technique-a method to measure the current through a single ion channel that was developed by two German researchers in the 1970s-in their study, predicting that they could use it as a tool for what they call "in vivo, time-resolved structural biology".
Grosman also relied on an approach his lab evolved in 2005, showing that ionizable amino acids could be engineered into the inner lining of the channel pore, and that such changes to the amino acid sequence alter the current, which in turn reveals the structure of the open-channel conformation in unprecedented detail.
This approach allowed Grosman's team to map the relative position of every amino acid that formed the ion channel.
Grosman compared the new findings with those of direct studies of the structure of the closed channel, and came to the conclusion that the conformational changes that allow the channel to open are quite subtle. The five subunits that make up the protein channel do not rotate or pivot dramatically when opening the gate.
"There are many good reasons why I think a subtle conformational change is advantageous from an evolutionary point of view," Nature magazine quoted Grosman as saying.
Muscle nicotinic receptors must respond to acetylcholine with staggering speed, opening within microseconds of their exposure to the neurotransmitter, he said.
"These ion channels are meant to be quick. If they are too slow, we have disease," he said.
Grosman claimed that, with this work, scientists in his lab had become the first to infer the structure of an ion channel in its open conformation as it functions in a living cell.
"I know when the protein is open, because in single-molecule experiments the distinction between open and closed conformations is simple; the channel either passes a current or not," he said.
In a living cell the protein responds, in measurable ways, to changes in its structure and environment, he said.
"It's not frozen at super low temperatures. It's not in a crystalline lattice. The cells are alive at the beginning of the experiment and when we finish the experiment, the cells keep living," added the researcher.