Biophysicists at the National Science Foundation’s National High Magnetic Field Laboratory in Tallahassee, Florida, have discovered that membrane proteins give rise to unique patterns of signals in their nuclear magnetic resonance (NMR) spectra. This result opens a new approach for the three dimensional characterization of membrane protein structures.
Membrane proteins are responsible for communication between the external cellular environment and the cell’s interior where chemistry and biological functions are typically accomplished. Membrane proteins are often responsible for cellular recognition and for the transport of nutrients into and products out of cells. However, these important proteins have been particularly difficult to characterize by standard technologies and hence few membrane protein structures are known today.
"About 25 percent of proteins are membrane proteins, yet structures of only few of these are known," says Kamal Shukla, director of NSF's molecular biophysics program, which funded the research. "X-ray crystallography and solution NMR cannot be used for these proteins because they are hard to crystallize and are not soluble. The methodology developed by Cross and his colleagues for obtaining structural information of integral membrane proteins is therefore exceedingly important."
It has been known for some time that structural constraints from solid state NMR spectroscopy of uniformly aligned samples can be used to develop a high resolution three-dimensional structure. However, while many constraints can be obtained there has been no approach for dependable resonance assignments. In other words, without knowing where in the molecule each signal comes from it has been difficult to make progress with structural characterization.
Now, researchers Tim Cross, Riqiang Fu and Jack Quine, and their coworkers, supported by the NSF’s molecular biophysics program, have discovered that the signal patterns observed in two dimensional spectra directly reflect the distribution of amino acids about a helical axis, known as a helical wheel. Through standard methods of isotopic labeling using bacterial cultures, it is now possible to assign these signals to specific atomic sites in the membrane protein helices.
Furthermore, the location of the resonance patterns in the spectrum defines the tilt of the helix within the membrane. Indeed, it is possible to get this topological information on a helix without signal assignments - the first time this has been possible in NMR spectroscopy.
These results have been published as the cover story in the May 2000 issue of the Journal of Magnetic Resonance.
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