Alpha-synuclein is a major constituent of pathological intracellular inclusion bodies, a common feature of several neurodegenerative diseases. Two missense mutations in the alpha-synuclein gene have been identified in confirmed autosomal-dominant familial Parkinson's disease, which segregate with the illness. However, the physiological function of alpha-synuclein remains unknown. After biochemical investigations we have revealed tubulin to be an alpha-synuclein associated/binding protein. Here, we show that alpha-synuclein induces polymerization of purified tubulin into microtubules. Mutant forms of alpha-synuclein lose this potential. The binding site of alpha-synuclein to tubulin is identified, and co-localization of alpha-synuclein with microtubules is shown in cultured cells. To our knowledge, this is the first demonstration of microtubule-polymerizing activity of alpha-synuclein. Now we can see a striking resemblance between alpha-synuclein and tau: both have the same physiological function and pathological features, making abnormal structures in diseased brains known as synucleinopathies and tauopathies. The discovery of a physiological role for alpha-synuclein may provide a new dimension in researches into the mechanisms of alpha-synuclein-associated neurodegenerative diseases.