The kinase PINK1 and the E3 ubiquitin (Ub) ligase Parkin participate in mitochondrial quality control. The phosphorylation of Ser65 in Parkin's ubiquitin-like (UBl) domain by PINK1 stimulates Parkin activation and translocation to damaged mitochondria, which induces mitophagy generating polyUb chain. However, Parkin Ser65 phosphorylation is insufficient for Parkin mitochondrial translocation. Here we report that Ser65 in polyUb chain is also phosphorylated by PINK1, and that phosphorylated polyUb chain on mitochondria tethers Parkin at mitochondria. The expression of Tom70^MTS-4xUb SE, which mimics phospho-Ser65 polyUb chains on the mitochondria, activated Parkin E3 activity and its mitochondrial translocation. An E3-dead form of Parkin translocated to mitochondria with reduced membrane potential in the presence of Tom70^MTS-4xUb SE, whereas non-phospho-polyUb mutant Tom70^MTS-4xUb SA abrogated Parkin translocation. Parkin binds to the phospho-polyUb chain through its RING1-In-Between-RING (IBR) domains, but its RING0-linker is also required for mitochondrial translocation. Moreover, the expression of Tom70^MTS-4xUb SE improved mitochondrial degeneration in PINK1-deficient, but not Parkin-deficient, Drosophila. Our study suggests that the phosphorylation of mitochondrial polyUb by PINK1 is implicated in both Parkin activation and mitochondrial translocation, predicting a chain reaction mechanism of mitochondrial phospho-polyUb production by which rapid translocation of Parkin is achieved. Parkinson's disease is a neurodegenerative disorder caused by degeneration of the midbrain dopaminergic system in addition to other nervous systems. PINK1 and parkin, which encode mitochondrial protein kinase and cytosolic Ub ligase, respectively, were identified as the genes responsible for the autosomal recessive form of juvenile Parkinson's disease. Activation of PINK1 upon reduction of mitochondrial membrane potential recruits Parkin from the cytosol activating its Ub ligase activity, which ensures removal of damaged mitochondria through mitophagy. However, how PINK1 recruits Parkin to the damaged mitochondria remained unclear. Here, we describe that the phosphorylation of polyUb chain by PINK1 is a key event to recruit Parkin on the mitochondria. Parkin binds to, and is activated by, phospho-polyUb generated by Parkin in collaboration with PINK1. Expression of a phospho-polyUb mimetic protein on mitochondria rescued mitochondrial degeneration caused by loss of PINK1 in Drosophila. Our study suggests the existence of an amplification cascade of Parkin activation and mitochondrial translocation, in which a ‘seed' of phosphorylated polyUb on the mitochondria, generated by PINK1 and Parkin, triggers a chain reaction of Parkin recruitment and activation.

UbC is one of two stress‐inducible polyubiquitin genes in mammals and is thought to supplement the constitutive UbA genes in maintaining cellular ubiquitin (Ub) levels during episodes of cellular stress. We have generated mice harboring a targeted disruption of the UbC gene. UbC −/− embryos die between embryonic days 12.5 and 14.5 in utero , most likely owing to a severe defect in liver cell proliferation. Mouse embryonic fibroblasts from UbC −/− embryos exhibit reduced growth rates, premature senescence, increased apoptosis and delayed cell‐cycle progression, with slightly, but significantly, decreased steady‐state Ub levels. UbC −/− fibroblasts are hypersensitive to proteasome inhibitors and heat shock, and unable to adequately increase Ub levels in response to these cellular stresses. Most, but not all of the UbC −/− phenotypes can be rescued by providing additional Ub from a poly hemagglutinin‐tagged Ub minigene expressed from the Hprt locus. We propose that UbC is regulated by a process that senses Ub pool dynamics. These data establish that UbC constitutes an essential source of Ub during cell proliferation and stress that cannot be compensated by other Ub genes.

Genetic ablation of autophagy in mice leads to liver and brain degeneration accompanied by the appearance of ubiquitin (Ub) inclusions, which has been considered to support the hypothesis that ubiquitination serves as a cis-acting signal for selective autophagy. We show that tissue-specific disruption of the essential autophagy genes Atg5 and Atg7 leads to the accumulation of all detectable Ub–Ub topologies, arguing against the hypothesis that any particular Ub linkage serves as a specific autophagy signal. The increase in Ub conjugates in Atg7−/− liver and brain is completely suppressed by simultaneous knockout of either p62 or Nrf2. We exploit a novel assay for selective autophagy in cell culture, which shows that inactivation of Atg5 leads to the selective accumulation of aggregation-prone proteins, and this does not correlate with an increase in substrate ubiquitination. We propose that protein oligomerization drives autophagic substrate selection and that the accumulation of poly-Ub chains in autophagy-deficient circumstances is an indirect consequence of activation of Nrf2-dependent stress response pathways.

Ubiquitin is encoded in mice by two polyubiquitin genes, Ubb and Ubc, that are considered to be stress inducible and two constitutively expressed monoubiquitin (Uba) genes. Here we report that targeted disruption of Ubb results in male and female infertility due to failure of germ cells to progress through meiosis I and hypogonadism. In the absence of Ubb, spermatocytes and oocytes arrest during meiotic prophase, before metaphase of the first meiotic division. Although cellular ubiquitin levels are believed to be maintained by a combination of functional redundancy among the four ubiquitin genes, stress inducibility of the two polyubiquitin genes, and ubiquitin recycling by proteasome-associated isopeptidases, our results indicate that ubiquitin is required for and consumed during meiotic progression. The striking similarity of the meiotic phenotype in Ubb^−/− germ cells to the sporulation defect in fission yeast (Schizosaccharomyces pombe) lacking a polyubiquitin gene suggests that a meiotic role of the polyubiquitin gene has been conserved throughout eukaryotic evolution.