A Bend In A Protein is Key to The Gating Mechanism of Channels Between Cells, UB Biologists Discover

BUFFALO, N.Y. -- Healthy cells throughout the body coordinate activities with their immediate neighbors by sending chemical and electrical signals across tiny passageways called gap-junction channels. When a cell's health is compromised, as a result of trauma or disease, healthy cells around it protect themselves by closing these channels and shutting off communication.

But while scientists have observed this in experiments, the mechanism responsible for the channels shutting down remained a mystery. Until now.

A group of biologists at the University at Buffalo report that they have identified an amino acid, called proline, that plays a central role in shutting down gap-junction channels.

"The identification of this amino acid, proline, gives us the first foot in the door to figuring out what causes the channels to shut down," said Bruce J. Nicholson, Ph.D., associate professor of biological sciences at UB, co-author and leader of a group studying the structure and function of these intercellular channels.

The research offers not only important information necessary to finding out how these particular channels may be shut down in certain disease states, but also may be the key to a more general mechanism of opening and closing other channels important in the nerve and muscle systems.

Published recently in Nature, the work was supported by the National Institutes of Health and a Pew Scholarship awarded to Nicholson.

"Understanding the gating mechanism is relevant because opening and closing of these channels is likely to be critical in modifying the rate of heart-muscle contraction and in understanding the initiation of tumor growth," Nicholson said.

The biologists investigated proline, an amino acid that produces a bend, or distortion, in one of the membrane-spanning portions of the gap-junction protein. This was found to represent a critical focal point for changes in the orientation of the protein that cause the channel to close in response to a difference in the voltage between cells.

"All cells exist at a negative voltage compared to the outside world," Nicholson explained, "so when two normal cells talk to each other, they are both equivalently negative and there is no voltage difference across the junction. However, if one cell suffers injury, its voltage will approach that of the outside of the cell. This causes a voltage difference between the cells, allowing the healthy cell to shut the channels down."

This is a protective mechanism, Nicholson noted.

"Not only would it be rather boring to continue talking to a dead neighbor, it would be lethal to any cell to share even portions of its contents with sick neighbors for any extended time," he said.

The team discovered proline's role in shutting down channels when it used genetic engineering to replace this amino acid with others that do not produce the characteristic bend in the protein. These mutants responded to voltage changes across the channel by opening, rather than closing, thus reversing the channel's gating response.

Thomas M. Suchyna, Ph.D., a postdoctoral fellow in Nicholson's laboratory and the lead author, noted that while this work demonstrates the role of proline in shutting down communication via gap-junction channels, it is also potentially relevant to other kinds of channels, such as those involved in transmission of electrical impulses between nerve cells and nerves and muscles.

"This amino acid also is found in a similar context in these other channels," said Suchyna. "Thus, it may represent a general mechanism of gating."

The group is currently studying proline's role in activating nervous-system ion channels with Anthony L. Auerbach, Ph.D., associate professor of biophysical sciences in the UB School of Medicine and Biomedical Sciences.

Other co-authors of the Nature paper are Charles R. Fourtner, Ph.D., UB professor of biological sciences; Lie Xian Xu, Ph.D., formerly a visiting professor at UB who is now at Nanjing University in the People's Republic of China, and Feng Gao, a research technician at UB.