Intracellular aggregation of polypeptides with expanded polyglutamine domain is stimulated by stress-activated kinase MEKK1

Abnormal proteins, which escape chaperone-mediated refolding or proteasome- dependent degradation, aggregate and form inclusion bodies (IBs). In severa l neurodegenerative diseases, such IBs can be formed by proteins with expan ded polyglutamine (polyQ) domains (e.g., huntingtin). This work studies the regulation of intracellular IB formation using an NH2-terminal fragment of huntingtin with expanded polyQ domain. We demonstrate that the active form of MEKK1, a protein kinase that regulates several stress-activated signali ng cascades, stimulates formation of the IBs. This function of MEKK1 requir es kinase activity, as the kinase-dead mutant of MEKK1 cannot stimulate thi s process. Exposure of cells to UV irradiation or cisplatin, both of which activate MEKK1, also augmented the formation of IBs. The polyQ-containing h untingtin fragment exists in cells in two distinct forms: (a) in a discrete soluble complex, and (b) in association with insoluble fraction. MEKK1 str ongly stimulated recruitment of polyQ polypeptides into the particulate fra ction. Notably, a large portion of the active form of MEKK1 was associated with the insoluble fraction, concentrating in discrete sites, and polyQ-con taining IBs always colocalized with them. We suggest that MEKK1 is involved in a process of IB nucleation. MEKK1 also stimulated formation of IBs with two abnormal polypeptides lacking the polyQ domain, indicating that this k inase has a general effect on protein aggregation.