Ubiquitin‐dependent and ‐independent mitochondrial protein quality controls: implications in ageing and neurodegenerative diseases

The ubiquitin‐independent protein quality control of matrix proteins of the mitochondrion is well characterized and until recently the mitochondrion was considered a ‘ubiquitination‐free’ organelle. However, a number of studies now indicate multiple roles of the ubiquitin–proteasome pathway in the regulation and maintenance of mitochondrial integrity. Of particular interest to this review is the finding of a mitochondrial ubiquitin‐dependent protein quality control and that this pathway may share similarity to the e ndoplasmic r eticulum‐a ssociated d egradation (ERAD) pathway that acts to eliminate misfolded proteins from the lumen of the endoplasmic reticulum. The potential cross‐talk between the ubiquitin‐dependent and ‐independent protein quality controls and their implications in ageing and neurodegenerative diseases, notably in Parkinson's disease, are discussed.