Biology subjects

Brandman, O.

Publications and source records attributed to Brandman, O..

4 recordsLinked to original sources

bioRxiv2019

Rqc2 levels alter CAT tail composition, function, and toxicity in S. cerevisiae

The Ribosome-associated Quality Control (RQC) pathway co-translationally marks incomplete polypeptides from stalled translation with two signals that trigger their proteasome-mediated degradation. The E3 ligase Ltn1 adds ubiquitin and Rqc2 directs the large ribosomal subunit to append carboxy-terminal alanine and threonine residues (CAT tails). When excessive amounts of incomplete polypeptides evade Ltn1, CAT-tailed proteins accumulate and can self-associate into aggregates. CAT tail aggregation has been hypothesized to either protect cells by sequestering potentially toxic incomplete polypeptides or harm cells by disrupting protein homeostasis. To distinguish between these possibilities, we modulated CAT tail aggregation in Saccharomyces cerevisiae with genetic and chemical tools to analyze CAT tails in aggregated and un-aggregated states. We found that enhancing CAT tail aggregation induces proteotoxic stress and antagonizes degradation of CAT-tailed proteins, while inhibiting aggregation reverses these effects. Our findings suggest that CAT tail aggregation harms RQC-compromised cells and that preventing aggregation can mitigate this toxicity.

cell biology↗

bioRxiv2019

Chaperone-Mediated Reflux of Secretory Proteins to the Cytosol During Endoplasmic Reticulum Stress

Diverse perturbations to endoplasmic reticulum (ER) functions compromise the proper folding and structural maturation of secretory proteins. To study secretory pathway physiology during such \"ER stress\", we employed an ER-targeted, redox-responsive, green fluorescent protein--eroGFP--that reports on ambient changes in oxidizing potential. Here we find that diverse ER stress agents cause properly folded, ER-resident eroGFP (and other ER luminal proteins) to \"reflux\" back to the reducing environment of the cytosol as intact, folded proteins. By utilizing eroGFP in a comprehensive genetic screen in S. cerevisiae, we show that ER protein reflux during ER stress requires specific chaperones and co-chaperones residing in both the ER and the cytosol. Chaperone-mediated ER protein reflux does not require E3 ligase activity, and proceeds even more vigorously when these ER-associated degradation (ERAD) factors are crippled, suggesting that reflux may work in parallel with ERAD. In summary, chaperone-mediated ER-protein reflux may be a conserved protein quality control process that evolved to maintain secretory pathway homeostasis during ER protein-folding stress.\n\nSIGNIFICANCEApproximately one third of eukaryotic proteins are synthesized on ribosomes attached to the endoplasmic reticulum (ER) membrane. Many of these polypeptides co- or post-translationally translocate into the ER, wherein they fold and mature. An ER quality-control system proofreads these proteins by facilitating their folding and modification, while eliminating misfolded proteins through ER-associated degradation (ERAD). Yet, the fate of many secretory proteins during ER stress is not completely understood. Here, we uncovered an ER-stress induced \"protein reflux\" system that delivers intact, folded ER luminal proteins back to the cytosol without degrading them. We found that ER protein reflux works in parallel to ERAD and requires distinct ER-resident and cytosolic chaperones and co-chaperones.

cell biology↗

bioRxiv2019

MISTERMINATE Mechanistically Links Mitochondrial Dysfunction with Proteostasis Failure

Mitochondrial dysfunction and proteostasis failure frequently coexist as hallmarks of neurodegenerative disease. How these pathologies are related is not well understood. Here we describe a phenomenon termed MISTERMINATE (mitochondrial stress-induced translational termination impairment and protein carboxyl terminal extension), which mechanistically links mitochondrial dysfunction with proteostasis failure. We show that mitochondrial dysfunction impairs translational termination of nuclear-encoded mitochondrial mRNAs including complex-I 30kD subunit (C-I30) mRNA, occurring on mitochondrial surface in Drosophila and mammalian cells. Ribosomes stalled at the normal stop codon continue to add to the C-terminus of C-I30 certain amino acids non-coded by mRNA template. C-terminally-extended C-I30 is toxic when assembled into C-I and forms aggregates in the cytosol. Enhancing co-translational quality control prevents C-I30 C-terminal extension and rescues mitochondrial and neuromuscular degeneration in a Parkinsons disease model. These findings emphasize the importance of efficient translation termination and reveal unexpected link between mitochondrial health and proteome homeostasis mediated by MISTERMINATE.

cell biology↗

bioRxiv2018

CAT tails drive on- and off-ribosome degradation of stalled polypeptides

Stalled translation produces incomplete, ribosome-associated polypeptides that Ribosome-associated Quality Control (RQC) targets for degradation via the ubiquitin ligase Ltn1. During this process, the Rqc2 protein and large ribosomal subunit elongate stalled polypeptides with carboxy-terminal alanine and threonine residues (CAT tails). Failure to degrade CAT-tailed proteins disrupts global protein homeostasis, as CAT-tailed proteins aggregate and sequester chaperones. Why cells employ such a potentially toxic process during RQC is unclear. Here, we developed quantitative techniques to assess how CAT tails affect stalled polypeptide degradation in Saccharomyces cerevisiae. We found that CAT tails improve Ltn1s efficiency in targeting structured polypeptides, which are otherwise poor Ltn1 substrates. If Ltn1 fails, CAT tails undergo a backup route of ubiquitylation off the ribosome, mediated by the ubiquitin ligase Hul5. Thus, CAT tails functionalize the carboxy-termini of stalled polypeptides to drive their degradation on and off the ribosome.

cell biology↗