Mutations in the kinase PINK1 and the E3 ubiquitin ligase Parkin are associated with early-onset Parkinson's disease. PINK1/Parkin-mediated mitophagy is thought to ensure mitochondrial quality control in neurons as well as other cells. Upon the loss of mitochondrial membrane potential, lysine 63 (K63)-linked polyubiquitin chains accumulate on the mitochondrial outer membrane in a Parkin-dependent manner. However, the physiological significance of K63-linked polyubiquitination during mitophagy is not fully understood. Here, we report that the suppression of K63-linked polyubiquitination through the removal of Ubc13 activity essentially affects neither PINK1 activation nor the degradation of depolarized mitochondria.
Loss of Ubc13 markedly decreased the accumulation of both total polyubiquitin and K63-linked polyubiquitin chains on the mitochondria. However, Parkin was translocated to the mitochondria and subsequently degraded the mitochondrial proteins with the same efficiency upon the loss of mitochondrial membrane potential. In addition, PINK1 autophosphorylation, which is required for its kinase activation, occurred to the same extent in the presence or absence of Ubc13 activity. Finally, mitochondrial degeneration caused by PINK1 inactivation was not modulated by the inhibition of Ubc13 activity in Drosophila. Our data indicate that the formation of K63-linked polyubiquitin chains on depolarized mitochondria is not a key factor for PINK1/Parkin-mediated mitophagy as was once thought.