Cell’s Water Channel Gets A Close-Up

Moving water across cell membranes in a quick and controlled fashion is crucial to most life-forms, and it’s a job done with dispatch by a family of membrane proteins called aquaporins. A team of researchers led by structural biologist Richard Neutze of the University of Gothenburg, in Sweden, has now solved the structure of a yeast aquaporin at 0.88 Å, the highest resolution structure ever obtained for any membrane protein (Science 2013, DOI: 10.1126/science.1234306).

Although many aquaporin structures have been solved over the past 15 years, the sharper image will help researchers better understand the basic biology and mechanism of the channel, the role of aquaporin in disease, and how to use the membrane channel to desalinate ocean water.

The extremely high resolution was pure serendipity, explains Neutze. Back in 2009, his group published an X-ray crystal structure of aquaporin at 1.15 Å—a previous record for membrane proteins. At the time, he asked his team to make crystals of aquaporin suitable for neutron diffraction analysis to get new insight on the protein’s structure. For neutron diffraction studies, the protein crystals have to be large and need to be doped with deuterium instead of hydrogen. “We got large crystals,” he says. However, there wasn’t enough deuterated protein for neutron diffraction.

Instead of ditching the crystals intended for neutron diffraction, the team tried bombarding them with X-rays. The team then found that the crystals diffracted X-rays with enough precision to achieve yet another resolution record.

“Getting around 2-Å resolution is already very good for membrane proteins,” comments Jochen S. Hub, who studies aquaporins at the University of Göttingen, in Germany. Neutze and coworkers “got tremendously high resolution. You can see hydrogens, electron-withdrawing groups, double bonds—all typically unseen in membrane proteins,” he says. “It’s extremely impressive.”

Hub notes that the structure may enable research aimed at finding inhibitors for some of the 13 aquaporins found in humans. For example, in glaucoma, one version of the protein, called aquaporin-1, transports too much water, leading to pressure in the eyes. And aquaporin-4 is a problem in the brain after head traumas, such as from car accidents, because it also allows the transport of water leading to fatal brain swelling.

All aquaporins have six α-helices that span the cell membrane as well as a seventh pseudo-transmembrane helix, so called because it is formed by two half-helices that begin on opposite sides of the membrane and meet in the middle, Neutze explains. The new structure enables 10 water molecules to be viewed in the channel and shows how some of these waters form hydrogen bonds with amino acid residues long known to be critical to water transport. The structure also suggests that the water molecules traverse the channel in pairs.

“When I first saw the structure, I thought, ‘Wow, this looks like a potassium channel,’ ” comments Jeff Abramson, a membrane channel crystallographer at the University of California, Los Angeles. In particular, Abramson says, the newly visible location of water molecules is similar to the way potassium ions traverse ion channels, by hopping in pairs from binding site to binding site. “The aquaporins and potassium channels have completely different structures, but they’re using the same mechanism. It’s a cool case of convergent evolution,” he adds.

The structure also confirms work done in molecular dynamics simulations about how the channel is selective for water but not OH^– or H₃O⁺ ions, Hub adds. For example, the structure suggests that H₃O⁺ ions are excluded because of electrostatic repulsion from several positively charged amino acid side chains in the channel and the dipole moment of the two half-helices, Neutze says.