Aquaporins are a bit like bouncers at a club. These proteins span the cell membrane, forming an hourglass-shaped core that allows water molecules to enter (right), but excludes ions, such as protons or hydroxide. Without aquaporins, cells wouldn't be able to properly regulate their water intake and salt balance, and diseases such as some forms of diabetes and bipolar disorder can result when there are defects in these channels. Now, researchers have created the highest resolution crystal structure of any membrane protein—an aquaporin from yeast—and revealed the trick to its selectivity. The structure (left), reported online today in Science, shows how amino acids in the protein's core (orange and purple cylinders) use hydrogen bonds that they share with water molecules (red circles) to orient the waters in just such a way to allow them to zip through in pairs (black arrows). And because charged ions can't form these same bonding arrangements, they are shut out. Hey, not everyone gets past the velvet rope.
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