Very interesting, and simple English, article on alpha-synuclein in National Geographic:

We talk about Parkinson’s disease rather than Parkinson’s diseases, as if this disorder was a single thing. It is and it isn’t. Any two patients can differ greatly in when they first showed symptoms, what problems they experience, and how quickly their condition worsens. Some people only ever have difficulties with movement, while others suffer from full-blown dementia.

And yet, every case of Parkinson’s boils down to a protein called alpha-synuclein. Think of a protein as a sculpture made by folding up a long chain of beads. If alpha-synuclein folds correctly, it helps the neurons in our brain to send messages to one another. If it folds incorrectly, it becomes rowdy and sociable, gathering in large destructive clumps that wreck neurons.

http://phenomena.nationalgeographic....of-parkinsons/