Proteins tagged with lysine (Lys, K) 48 polyubiquitins chains are destined for 
degradation by the 26S proteasomal system. Impairment of the ubiquitin 
proteasome system (UPS) function culminates in the accumulation of 
polyubiquitinated proteins in many neurodegenerative conditions including 
Parkinson's disease (PD). Nevertheless, the cellular mechanisms underlying cell 
death induced by an impaired UPS are still not clear. Intriguingly, recent 
studies indicate that several proteins associated with familial PD are capable 
of promoting the assembly of Lys-63 polyubiquitin chains. Therefore, the 
objective of this study was to examine the role of K48 and K63 ubiquitination in 
mitochondria-mediated apoptosis in in vitro models of dopaminergic degeneration. 
Exposure of the widely used proteasome inhibitor MG-132 to dopaminergic neuronal 
cell line (N27) induced a rapid accumulation of polyubiquitinated proteins in 
the mitochondria. This appears to result in the preferential association of 
ubiquitin conjugates in the outer membrane and polyubiquitination of outer 
membrane proteins. Interestingly, the ubiquitin(K48R) mutant effectively rescued 
cells from MG-132-induced mitochondrial apoptosis without altering the 
antioxidant status of cells; whereas the ubiquitin(K63R) mutant augmented the 
proapoptotic effect of MG-132. Herein, we report a novel conclusion that 
polyubiquitinated proteins, otherwise subjected to proteasomal degradation, 
preferentially accumulate in the mitochondria during proteolytic stress; and 
that polyubiquitination of Lys-48 and Lys-63 are key determinants of 
mitochondria-mediated cell death during proteasomal dysfunction. Together, these 
findings yield novel insights into a crosstalk between the UPS and mitochondria 
in dopaminergic neuronal cells.