Light Triggers Nanovalve

MOLECULAR DEVICES

By chemically tinkering with a well-studied channel protein, researchers in the Netherlands have created a membrane-bound nanovalve that can be opened and closed with light (Science 2005, 309, 755). The scientists say this switchable pore could potentially be used to transport and deliver drugs or other substances.

"A nanovalve can be considered a key element in the nanotoolbox that will be needed to ultimately construct nanomachinery and devices," says Ben L. Feringa, the University of Groningen chemistry professor who designed and built the nanovalve along with graduate student Martin Walko and BioMaDe Technology Foundation scientists Armag(breve)an Koçer and Wim Meijberg. "Think of the numerous channels and valves in our cells with a diversity of functions in controlled transport in and out of the cell."

The Dutch team found the inspiration for their nanovalve in science's workhorse cell, the bacterium Escherichia coli. The mechanosensitive channel of large conductance, or MscL for short, is one of E. coli's best characterized channel proteins. It's made up of five identical subunits, and it acts as a safety valve for the microbe. Should the internal pressure of an E. coli bacterium become too high, these channel proteins open up and release fluid, ions, and small proteins, preventing the microbe from exploding.

Previously, scientists showed that MscL will spontaneously open if charged amino acids or other molecules are attached to the 22nd residue on each of the protein's five subunits. Presumably, water attracted to these hydrophilic substituents interferes with hydrophobic interactions that keep the pore closed.

The Dutch researchers reasoned that if they modified this "sweet spot" with a small molecule that responds to light by switching from a neutral form to a charged form, they could create a channel that opens and closes on command.

To incorporate their molecular switch, the researchers replaced the critical residue with cysteine, an amino acid that's not present anywhere else in the protein. They then selectively attached a small, light-sensitive molecule to the cysteines. The modification was made at each subunit, ultimately adding five switches to the channel.

According to Walko, when the molecule is irradiated with long-wavelength ultraviolet light, it isomerizes from a neutral spiropyran to a zwitterionic merocyanine via an electrocyclic ring opening. The charged merocyanine group causes the channel protein to open. Visible light reverses the reaction and closes the channel. "The key here is that we can use light to externally trigger the opening and closing of a membrane pore," Feringa explains.

To see if their system would work within a cell membrane, the researchers embedded the modified protein in a synthetic lipid membrane. While the pore did open and close in response to light, the researchers note that the switching becomes less efficient after the first cycle. Feringa says they are currently trying to figure out why this happens so that they can improve the nanovalve and incorporate it into drug delivery systems.

"It's amazing, isn't it, what ingenious chemists can do when they start tinkering in a logical way with nature?" remarks nanomachine expert J. Fraser Stoddart of the University of California, Los Angeles. "The latest nanovalve to come out of the Feringa laboratory is an exquisite marriage of a biological system with a chemical one."