Abnormally modified alpha-synuclein is a pathological hallmark of Parkinson's disease and the other alpha-synucleinopathies. Since proteinase K (PK) treatment is known to enhance the immunoreactivity of abnormal alpha-synuclein, we immunohistochemically examined the brain of transgenic (Tg) mice expressing human mutant A53T alpha-synuclein using this retrieval method. PK treatment abolished the immunoreactivity of alpha-synuclein in abnormal inclusions as well as of endogenous alpha-synuclein in Tg mice, whereas PK-resistant alpha-synuclein was found in the presynaptic nerve terminals, especially in the hippocampus and temporal cortex. In human Lewy body disease, PK-resistant alpha-synuclein was deposited in Lewy bodies and Lewy neurites, as well as in the presynapses in distinct brain regions, including the hippocampus, temporal cortex and substantia nigra. Biochemical analysis revealed that PK-resistant alpha-synuclein was detected in the presynaptic fraction in Tg mice and human Lewy body disease. Although PK-resistant alpha-synuclein was found in the presynapse in Tg mice even at 1 week of age, it was not phosphorylated until at least 8 months of age. Moreover, PK-resistant alpha-synuclein in the presynapse was not phosphorylated in human Lewy body disease. These findings suggest that phosphorylation is not necessary to cause the conversion of soluble form to PK-resistant alpha-synuclein. Considering that native alpha-synuclein is a soluble protein localized to the presynaptic terminals, our findings suggest that PK-resistant alpha-synuclein may disturb the neurotransmission in alpha-synucleinopathies.