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HHMI investigator Christopher Miller says each new membrane protein structure teaches scientists something.
HHMI investigator Eric Gouaux, for example, recently found surprising links between the structures of different membrane proteins in the brain. The cells of the brain must pass along signals, in the form of neurotransmitters, in mere milliseconds. Neurotransmitters must be released from one cell, detected by another, and then mopped up by one of the cells so that the signaling can take place again. This happens entirely at the membranes of the neurons. Understanding how brain receptors work is vital to explaining learning, memory, and cognitive disorders.
Using x-ray crystallography, Gouaux and his lab team at Oregon Health & Science University have shown unexpected similarities between different types of neurotransmitter receptors. While the receptors have vastly different amino acid sequences, which suggests different structures, his group found that each receptor forms a similar shape. Piecing out these unexpected details will help scientists better predict the shape of membrane proteins from their amino acid sequences.
“At some point it will happen, maybe 10 years from now, that the next membrane protein structure will come out and everyone will yawn because it starts to feel like stamp collecting,” says Miller, at Brandeis University. But that hasn't happened yet, he says. “ The number of structures is still small enough that each one has something novel to tell people about how these things are put together, how they sit in the membrane, or how they fold.”
Miller knows well, though, that sometimes it pays to look beyond the structure. When MacKinnon published the structure of a chloride channel (CLC) in 2002, scientists thought they knew the basics of how it worked—it appeared to be a typical channel. But Miller took a closer look at the electrical behavior of the protein and found, much to everyone's astonishment, that this particular CLC isn't a passive channel. It's a pump that uses protons to push chloride across the membrane.
“It ended up being somewhat of a game-changer,” says Miller, “because within a year, other labs following our discovery tested the electrical properties of other CLCs and found that this family of CLCs is split into two subtypes.” Some CLCs are channels; others are pumps. The structures hadn't hinted at this. Pairing the function and structure of the proteins illuminated the full story of the CLC protein family.
As the list of membrane protein structures grows, and lessons are learned, scientists have also begun to toy more with other aspects of these proteins. Most notably: how do membrane proteins interact with, and depend on, the membrane that surrounds them?
If a single-cell bacterium, full of molecules and salts, fell into a puddle of water, the water would rush through the membrane into the cell. If this went on for too long, the membrane would rupture, unable to stretch very far. Luckily for the cell, it has a way to bail itself out.
Doug Rees, an HHMI investigator at California Institute of Technology, studies one type of stretch-activated, or mechanosensitive, channel. When these channels sense a membrane stretching too far, Rees says, they open, like the safety valve on a pressure cooker. Their function depends on the membrane around them. Rees focuses on bacterial versions, but stretch-activated channels play a role in maintaining the membrane's tension in all organisms. In humans, other mechanosensitive channels in the nervous system help convey a sense of touch by passing along a signal when the pressure on a membrane changes.
To study these channels, Rees inserts them into an artificial membrane and sucks up part of the membrane with a glass pipette, increasing membrane tension around the channels. As the tension increases, channels open and he can measure the molecules flowing through. His system is a simple way to probe how membrane proteins can detect and respond to one type of variation—tension— in the membrane.
Ultimately, Rees wants to determine the structures of these channels. Although he published the structure of one stretch-activated channel in Nature on September 3, 2009, Rees is frustrated by the limitations of typical x-ray crystallography for studying membrane proteins and wants to find a better method. In 2008, he received a Collaborative Innovator Award from HHMI to make membrane protein crystals with collaborators at Caltech and the University of Colorado. His idea is to coax proteins into forming polyhedral arrangements embedded in membranes. Such a shape—inspired by symmetrical 20-sided viruses—would allow proteins to interact normally with the membrane while still providing the sample homogeneity needed for x-ray crystallography.
Photo: Yiling Fang