α-synuclein is a protein found abundantly throughout the brain. It is present mainly at the neuron ends where it is thought to play a role in ensuring the supply of synaptic vesicles in presynaptic terminals, which are required for the release of neurotransmitters to relay signals between neurons. It is critical for normal brain function.
However, α-synuclein is also the primary protein component of the cerebral amyloid deposits characteristic of Parkinson’s disease and its precursor is found in the amyloid plaques of Alzheimer's disease. Although α-synuclein is present in all areas of the brain, these disease-state amyloid plaques only arise in distinct areas.
Alpha-synuclein protein. May play role in Parkinson's and Alzheimer's disease. © molekuul.be/shutterstock.com
Imaging of isolated samples of α-synuclein in vitro indicate that it does not have the precise 3D folded structure usually associated with proteins. It is therefore classed as an intrinsically disordered protein. However, it was not known whether the protein also lacked a precise structure in vivo.
There have been reports that it can form helical tetramers. Since the 3D structure of a biological protein is usually precisely matched to the specific function it performs, knowing the structure of α-synuclein within a living cell will help elucidate its role and may also improve understanding of the disease states with which it is associated.
If α-synuclein remains disordered in vivo, it may be possible for the protein to achieve different structures, and have different properties, depending on its surroundings.
Techniques for determining protein structure
It has long been known that elucidating the structure of a protein at an atomic level is fundamental for understanding its normal function and behavior. Furthermore, such knowledge can also facilitate the development of targeted drug treatments. Unfortunately, observing the atomic structure of a protein in vivo is not straightforward.
X-ray diffraction is the technique usually adopted for visualizing structures at atomic resolution, but this requires crystals of the molecule to be produced and this cannot be done without separating the molecules of interest from their natural environment. Such processes can modify the protein from its usual state and, particularly with complex structures, such effects are difficult to predict.
The development of nuclear magnetic resonance (NMR) spectroscopy improved the situation by making it possible for molecules to be analyzed under in vivo conditions, i.e. same pH, temperature and ionic concentration.
More recently, increases in the sensitivity of NMR and the use of isotope labelling have enabled determinations of the atomic level structure and dynamics of proteins to be determined within living cells1. NMR has been used to determine the structure of a bacterial protein within living cells2 but it is difficult to achieve sufficient quantities of the required protein within mammalian cells and to keep the cells alive for NMR imaging to be conducted.
Electron paramagnetic resonance (EPR) spectroscopy for determining protein structure
Recently, researchers have managed to overcome these obstacles by using in-cell NMR and electron paramagnetic resonance (EPR) spectroscopy. EPR spectroscopy is a technique that is similar to NMR spectroscopy in that it is based on the measurement and interpretation of the energy differences between excited and relaxed molecular states.
In EPR spectroscopy it is electrons that are excited, whereas in NMR signals are created through the spinning of atomic nuclei. EPR was developed to measure radicals and metal complexes, but has also been utilized to study the dynamic organization of lipids in biological membranes3.
© Bruker
EPR has now been used for the first time in protein structure investigations and has provided atomic-resolution information on the structure of α-synuclein in living mammalians4,5.
Bacterial forms of the α-synuclein protein labelled with 15N isotopes were introduced into five types of mammalian cell using electroporation. Concentrations of α-synuclein close to those found in vivo were achieved and the 15N isotopes allowed the protein to be clearly defined from other cellular components by NMR. The conformation of the protein was then determined using electron paramagnetic resonance (EPR).
The results showed that within living mammalian cells α-synuclein remains as a disordered and highly dynamic monomer. Different intracellular environments did not induce major conformational changes.
Summary
The novel use of EPR spectroscopy has resolved the mystery surrounding the in vivo conformation of α-synuclein. It showed that α-synuclein maintains its disordered monomeric form under physiological cell conditions. It has been demonstrated for the first time that even in crowded intracellular environments α-synuclein does not form oligomers, showing that intrinsic structural disorder can be sustained within mammalian cells.
References
- Freedberg DI and Selenko P. Live cell NMR Annu. Rev. Biophys. 2014;43:171–192.
- Sakakibara D, et al. Protein structure determination in living cells by in-cell NMR spectroscopy. Nature 2009;458:102–105.
- Yashroy RC. Magnetic resonance studies of dynamic organisation of lipids in chloroplast membranes. Journal of Biosciences 1990;15(4):281.
- Alderson TA and Bax AD. Parkinson’s Disease. Disorder in the court. Nature 2016; doi:10.1038/nature16871.
- Theillet FX, et al. Structural disorder of monomeric α-synuclein persists in mammalian cells. Nature 2016; doi:10.1038/nature16531.
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