α-synuclein retains a disordered monomeric form even within living mammalian cells
For many years, there has been controversy surrounding the structure of α-synuclein. This protein is classed as an intrinsically disordered protein since it was found not to have the precise 3D folded structure usually associated with proteins. However, it has only been viewed in vitro and some researchers reported that α-synuclein forms helical tetramers under physiological conditions. Consequently, it was proposed that α-synuclein may have a precise conformation in vivo that is lost on extraction.
There is particular interest in the α-synuclein protein since it was discovered to be a key component of the amyloid plaques that develop in the brain in Parkinson’s disease. Although the precise function of α-synuclein has yet to be fully elucidated, it is known to play a role in neurotransmission and be vital for normal brain function. It is therefore not surprising that α-synuclein is found throughout the brain.
What is surprising is that in Parkinson’s disease it only aggregates to form the characteristic plaques in very specific distinct areas of the brain. It was thought that the protein must therefore undergo conformational changes according to its immediate environment. The difficulties in viewing the protein in living mammalian cells have meant that such theories regarding the in vivo structure of α-synuclein could not be verified.
Researchers have now, for the first time, succeeded in visualizing the atomic structure of α-synuclein in living mammalian cells1,2. They achieved this by utilizing in-cell nuclear magnetic resonance (NMR) imaging with a similar technique that was developed to measure radicals and metal complexes — electron paramagnetic resonance (EPR) spectroscopy.
Like NMR, EPR images are achieved from the energy released during the transition from excited to relaxed molecular states. The difference being that in EPR it is the electrons that are made to spin, rather than atomic nuclei. EPR has recently started to be used in biological applications, for example, in the study of the dynamic organization of lipids in biological membranes3, but this is the first time it has been used in the determination of protein structure.
In five types of living mammalian cells the α-synuclein protein was present as a disordered, highly dynamic monomer. Major conformational changes were not observed between different intracellular environments.
The novel use of EPR spectroscopy has started to resolve the debate surrounding the in vivo conformation of α-synuclein. It has showed that proteins of intrinsic structural disorder do exist within mammalian cells under physiological cell conditions.
This research paves the way for further research into α-synuclein and the cause of Parkinsonian amyloid plaques, which may allow curative treatments to be developed in the future.
References
Theillet FX, et al. Structural disorder of monomeric α-synuclein persists in mammalian cells. Nature 2016; doi:10.1038/nature16531.
Alderson TA and Bax AD. Parkinson’s Disease. Disorder in the court. Nature 2016; doi:10.1038/nature16871.
Yashroy RC. Magnetic resonance studies of dynamic organisation of lipids in chloroplast membranes. Journal of Biosciences 1990;15(4):281.