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Cells direct membrane traffic by channel width

For a glycerol molecule, a measly angstrom's difference in diameter is a road-closed sign: You can't squeeze through unless you are a sleek, water-molecule-sized sports car, say scientists at the University of Illinois at Urbana-Champaign.

The roadway is in aquaporins, a class of proteins that form trans-membrane channels in cell walls in all forms of life. They allow for water movement between the cell and its environment. A subfamily of aquaporins allows slightly larger molecules, such as glycerol, to pass, too. In humans, 11 aquaporins have been identified, mostly in the kidney, brain and lens of the eye. Impaired function has been implicated in a variety of diseases.

Aquaporins are a target of scrutiny for the Theoretical and Computational Biophysics Group at the Beckman Institute for Advanced Science and Technology.

Using steered molecular dynamics, Beckman researchers have solved a mystery that years of protein crystallography couldn't accomplish. Reporting in the August issue of Structure, they show that the main structural difference that makes an aquaporin a glycerol channel is a channel that is just a hundred-millionth of a centimeter -- an angstrom -- wider than a normal water channel.

So even if glycerol molecules line up properly, as do water molecules to pass through a pure water channel (as documented by researchers in the same lab in 2002), the slightly larger sugar molecule is out of luck. The point of entry, known as a selectivity filter, is the most narrow, but there are other tight barriers blocking the way as well, said Emad Tajkhorshid, assistant director of research in the Beckman lab.

"Membrane proteins are difficult to crystallize," he said. "We don't have the known structure of many of them. There has been a lot of recent progress, and for aquaporins we've got four structures available, which is really exceptional for membrane channels."

For the new study, his team focused on two of them. "Both
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Source:University of Illinois at Urbana-Champaign


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