Maintaining the essential functions of mitochondria requires mechanisms to recognize and remove misfolded proteins. However, quality control (QC) pathways for misfolded mitochondrial proteins remain poorly defined. Here, we establish temperature-sensitive (ts-) peripheral mitochondrial outer membrane (MOM) proteins as novel model QC substrates in Saccharomyces cerevisiae. The ts- proteins sen2-1HAts and sam35-2HAts are degraded from the MOM by the ubiquitin-proteasome system. Ubiquitination of sen2-1HAts is mediated by the ubiquitin ligase (E3) Ubr1, while sam35-2HAts is ubiquitinated primarily by San1. Mitochondria-associated degradation (MAD) of both substrates requires the SSA family of Hsp70s and the Hsp40 Sis1, providing the first evidence for chaperone involvement in MAD. In addition to a role for the Cdc48-Npl4-Ufd1 AAA-ATPase complex, Doa1 and a mitochondrial pool of the transmembrane Cdc48 adaptor, Ubx2, are implicated in their degradation. This study reveals a unique QC pathway comprised of a combination of cytosolic and mitochondrial factors that distinguish it from other cellular QC pathways.
Protein degradation by the ubiquitin-proteasome system is essential to many processes. We sought to assess its involvement in the turnover of mitochondrial proteins in Saccharomyces cerevisiae. We find that deletion of a specific ubiquitin ligase (E3), Psh1p, increases the abundance of a temperature-sensitive mitochondrial protein, mia40-4pHA, when it is expressed from a centromeric plasmid. Deletion of Psh1p unexpectedly elevates the levels of other proteins expressed from centromeric plasmids. Loss of Psh1p does not increase the rate of turnover of mia40-4pHA, affect total protein synthesis, or increase the protein levels of chromosomal genes. Instead, psh1Δ appears to increase the incidence of missegregation of centromeric plasmids relative to their normal 1:1 segregation. After generations of growth with selection for the plasmid, ongoing missegregation would lead to elevated plasmid DNA, mRNA, and protein, all of which we observe in psh1Δ cells. The only known substrate of Psh1p is the centromeric histone H3 variant Cse4p, which is targeted for proteasomal degradation after ubiquitination by Psh1p. However, Cse4p overexpression alone does not phenocopy psh1Δ in increasing plasmid DNA and protein levels. Instead, elevation of Cse4p leads to an apparent increase in 1:0 plasmid segregation events. Further, 2 μm high-copy yeast plasmids also missegregate in psh1Δ, but not when Cse4p alone is overexpressed. These findings demonstrate that Psh1p is required for the faithful inheritance of both centromeric and 2 μm plasmids. Moreover, the effects that loss of Psh1p has on plasmid segregation cannot be accounted for by increased levels of Cse4p.
Mutations in the Cl-/H+ exchanger CLC7 and its subunit OSTM1 result in osteopetrosis, lysosomal disorders, and pigmentation defects in mice and humans. How CLC7/OSTM1 regulates pigmentation in skin and hair melanocytes remains unexplored. In human epidermal melanocytes, we found CLC7/OSTM1 localized to melanosomes, the organelles in which melanin is synthesized, where it negatively regulates melanin production. Using a novel ratiometric melanosomal pH indicator, we showed that CLC7 acidifies melanosomes, opposing the function of the oculocutaneous albinism II (OCA2) Cl- ion channel. The de novo CLC7 variant (CLC7-Y715C) that causes albinism in humans and mice, decreased melanocytes pigmentation, which was restored by coexpression of OCA2. Remarkably, the enlarged hyperacidic vacuoles caused by CLC7-Y715C were also rescued by OCA2 coexpression in both melanocytes and non-melanocytic cells. Our data uncover a novel mechanism by which CLC7 regulates melanocyte pigmentation and identifies OCA2 as a tool to counteract the effects of CLC7 activating mutations.
Retrovirus‐like transposon mRNA encodes envelope and replication proteins (gag and pol), and serves as genomic material for virus‐like particles (VLPs), a cytosolic structure in which the element's genome is reverse transcribed. In Saccharomyces cerevisiae, there are several families of such retrotransposons, including the abundant Ty1 element. Previously, genome‐wide screens have identified candidate genes that mediate Ty1 mobility. One candidate, RTT105, was identified as a negative regulator of Ty1 mobility. Because RTT105 contains no conserved domains, the nature of RTT105's role in Ty1 retrotransposition remains unknown. Rtt105p localizes at Ty1 RNA and Gag foci, suggesting a role in ribonucleoprotein complexes. Here, we use RNA‐Seq analysis of isogenic WT and RTT105 knockout strains to determine changes in gene expression that may explain RTT105's regulation of Ty1. In rtt105Δ, we identified significant down regulation of signal recognition particle (SRP), a conserved chaperone that binds ribosome‐nascent chain complexes for post‐transcriptional processing. SRP aids in translocating Gag to the ER where it encapsulates Ty1 RNA in trans at presumptive VLP assembly sites. Because endogenous Ty1 mobility is maintained at low levels by the host cell, our data suggest that RTT105 may support SRP‐dependent processing of Ty1. Deletion of RTT105 may induce down regulation of SRP‐dependent processing of Ty1, thereby decreasing genome‐wide mobility of the element. This result illuminates an important step in retrotransposon processing and regulation, and suggests at least one route by which Ty1 activity is actively maintained by the cell.This research was funded by HHMI, NIH, and the Huck Institutes of the Life Sciences.
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