Probing the structure of membrane proteins
Membrane proteins play an essential role in cell function, cell-cell communication and a fundamental role in organism health; however, structural information about them is sparse even though they are encoded for by around 30 per cent of all genes.
Professor Chad Rienstra and his group at the University of Illinois, US, have developed new nuclear magnetic resonance (NMR) techniques to determine membrane protein structure and orientation and presented the results at the ACS (American Chemical Society) meeting in Philadelphia earlier this week.
Most of the protein structural data collected to date has been obtained from crystallised protein samples using X-ray crystallography techniques.
However, membrane proteins are particularly difficult to crystallise, often requiring detergents to solubilise them which then interfere with the crystallisation process.
While access to the powerful X-ray crystallography machines in nationalised research centres needed for protein crystallography experiments is improving, there are currently less than 20 independent structures of membrane proteins reported in the literature.
According to Prof. Rienstra, the new technique can be applied to the large range of membrane proteins and fibrils that are not amenable to solution-state NMR spectroscopy or X-ray crystallography.
The researchers combined custom-built spectrometers, probes and novel pulse sequences to develop the solid-state NMR method for probing protein chemistry and structure.
Solid-state NMR reduces the need for full crystallinity needed for x-ray crystallography while also removing the need for the molecules to be soluble as would be required for solution-state NMR.
The technique involves analysing the dipolar line shapes of signals to calculate the relative orientations of molecular fragments.
Using the technique, they managed to determine vector angle restraints for the majority of the 56-residue B1 immunoglobulin binding domain of protein GB1.
“Using this technique, we can more precisely define the fragments of the molecule, and how they are oriented. That allows us to define protein features and determine structure at the atomic scale,” said Prof. Rienstra.
“With increased speed and sensitivity, we can obtain very high resolution spectra. And, because we can resolve thousands of signals at a time - one for each atom in the sample - we can determine the structure of the entire protein.”
In order to further improve the sensitivity of the technique and accelerate data collection, the group is developing smaller rotors that can be spun at rates exceeding 25,000 rotations per second.
"In our experiments, we explore couplings between atoms in proteins," said Prof. Rienstra.
"Our goal is to translate genomic information into high-resolution structural information, and thereby be able to better understand the function of the proteins."
