Ion flow through membrane channels is dictated by particle size
Ion channels are small passageways that control the movement of electrically charged particles across a cell’s membrane. But they’re persnickety gatekeepers: Each channel allows only one kind of ion to flow through. Now, new research from Rockefeller University reveals the thermodynamics behind how at least one type of ion channel — the one dedicated to potassium — maintains its selectivity.
In order for neurons to send the electrical signals that speed through your body, they must maintain an electrical disparity between the inside of the cells and their surrounding environment. Although scientists have long known that ion channels are responsible for preserving this disparity, they had questions about how the channels discern the difference between ions like sodium and potassium, which both carry a single positive charge.
In a paper in PLoS Biology, Rod MacKinnon, John D. Rockefeller Jr. Professor and head of the Laboratory of Molecular Neurobiology and Biophysics, demonstrates that potassium channels discriminate among ions based on their size.
Potassium ions are slightly bigger than sodium ions — with a radius about one-third larger — and that greater volume means they have less charge distributed across their surface. “Potassium takes more space, but it also diffuses its charge over a bigger area, so potassium has lower charge density,” says Steve Lockless, a postdoctoral fellow in the MacKinnon lab and the paper’s first author.
To find out whether the potassium channel relies on size or its charge density to identify potassium and exclude sodium, Lockless, MacKinnon and postdoctoral fellow Ming Zhou played a trick on it. They separated potassium channels from the cell membranes and put them in a solution with barium, an ion roughly the same size as potassium but, with two positive charges, about the same charge density as sodium.
“If the channel recognized the charge density, then we would not expect barium to bind to the potassium channel,” Lockless says. “But we found the opposite.” Barium bound to the potassium channel, indicating to the researchers that the channel is detecting an ion’s size.
Lockless and MacKinnon, a Howard Hughes Medical Institute investigator, conclude that while sodium ions may bounce around at the channel’s opening, they are too small and their charge too concentrated to press all the right points for the potassium channel to conduct the ions through it.
Citation: PLoS Biology 5(5): e121 (May 1, 2007)
Source: Rockefeller University