[Impairment of the ubiquitin-proteasome system and neurodegeneration]

There are growing lines of evidence addressing the importance of the ubiquitin-proteasome system (UPS) that catalyzes various biological reactions rapidly, methodically, exhaustively, and unidirectionally. UPS is responsible for a diverse array of biologically important cellular processes, such as cell-cycle progression, signaling cascades and developmental programs. This system is also involved in the protein quality control, which maintains the homeostasis of the cell. Of particular interest is that UPS provides a clue for understanding of the molecular mechanisms underlying various neurodegenerative diseases. In the last decade, we witnessed a tremendous progress in uncovering the mechanisms of Parkinson's disease (PD). Of the several genes that can cause familial PD, parkin, the causative gene of autosomal recessive juvenile parkinsonism (AR-JP), is of a special interest because it encodes an ubiquitin-protein ligase, which covalently attaches ubiquitin to target proteins, designating them for destruction by the proteasome (a eukaryotic ATP-dependent protease complex). This review summarizes recent studies on the UPS pathway with a special reference to parkin, focusing on how parkin is linked to the pathogenesis of AR-JP.