PINK1, a mitochondrial serine/threonine kinase, is the product of a gene mutated in an autosomal recessive form of Parkinson disease. PINK1 is constitutively degraded by an unknown mechanism and stabilized selectively on damaged mitochondria where it can recruit the E3 ligase PARK2/PARKIN to induce mitophagy. Here, we show that, under steady-state conditions, endogenous PINK1 is constitutively and rapidly degraded by E3 ubiquitin ligases UBR1, UBR2 and UBR4 through the N-end rule pathway. Following precursor import into mitochondria, PINK1 is cleaved in the transmembrane segment by a mitochondrial intramembrane protease PARL generating an N-terminal destabilizing amino acid and then retrotranslocates from mitochondria to the cytosol for N-end recognition and proteasomal degradation. Thus, sequential actions of mitochondrial import, PARL-processing, retrotranslocation and recognition by N-end rule E3 enzymes for the ubiquitin proteosomal degradation defines the rapid PINK1 turnover. PINK1 steady-state elimination by the N-end rule identifies a novel organelle to cytoplasm turnover pathway that yields a mechanism to flag damaged mitochondria for autophagic elimination.
Keywords: PARKIN; PARL; mitochondrial import; mitophagy; ubiquitin.