Final answer:
Beta-amyloid is associated with plaques and tau with tangles in Alzheimer's, whereas alpha-synuclein forms toxic aggregates in Parkinson's disease. These protein aggregates lead to neurodegeneration.
Step-by-step explanation:
Considering the proteins involved in Alzheimer's and Parkinson's diseases, beta-amyloid is associated with plaques and tau protein is associated with neurofibrillary tangles in Alzheimer's disease. In the context of Parkinson's disease, the protein known as alpha-synuclein is linked to harmful aggregates within the brain cells, particularly in the substantia nigra nucleus. These misfolded protein aggregates interfere with normal cell function and contribute to the neurodegeneration seen in these diseases.