The molecular mechanisms of the mitoRQC pathway in the maintenance of mitochondrial homeostasis

• Applicant: Dr. Dejana Mokranjac, since 8/2019
• Subject Area: Biochemistry
• Term: from 2018 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 406246262

The cellular proteome requires a constant monitoring of the quality of its components since many factors lead to the production of aberrant toxic proteins. Cells contain a number of control pathways that mediate degradation of such faulty proteins or sequester them in aggregates. We have recently discovered a new control pathway that specifically protects mitochondria from accumulation of toxic proteins that are encoded by nuclear DNA and imported into the mitochondria. Surprisingly, such aberrant proteins are continually produced in normal respiring cells. This new pathway is specific for mitochondria and eliminates proteins while they are still present at ribosomes as peptidyl-tRNAs. Thus, this pathway belongs to the Ribosome-associated Quality Control pathways (RQC) and we named it mitoRQC. We identified so far three cytosolic proteins to have key functions in mitoRQC pathway in yeast: Vms1, Ltn1 und Rqc2. Vms1 was known as a poorly characterized protein that can prevent damage of mitochondria under stress conditions. Ltn1 is an E3-ligase that binds to 60 ribosomes (60S) and ubiquitylates lysine residues that are present in peptidyl-tRNAs directly at the exit tunnel of 60S. Rqc2 is a 60S-bound enzyme which, in an mRNA independent manner, adds Ala- and Thr-residues to the C-termini of peptidyl-tRNAs (CAT-tails). Our results suggest a model in which Vms1 and Ltn1 protect cells against toxic effects of faulty mitochondrial proteins while they are still being synthesized on the ribosomes. In the absence of Vms1 and Ltn1, Rqc2 adds CAT-tails to the faulty mitochondrial polypeptides stalled on 60S ribosomes. CAT-tailed polypeptides eventually get released from the 60S and imported into mitochondria where they aggregate, sequestering critical mitochondrial chaperones and translation machinery which inevitably leads to cell death. In this project, we aim at the deep molecular understanding of mitoRQC. We will combine biochemical approaches with yeast genetics and structural biology to understand the interaction of Vms1 with 60S ribosomes and its antagonistic action to Rqc2, to identify a possible receptor for Vms1 on the mitochondrial surface as well as to identify and characterize novel components of the mitoRQC. In addition, we will investigate how toxic polypeptides are cleared within mitochondria and which roles peptidyl-tRNA hydrolysis and ROS have in the mitoRQC. We expect that these experiments will allow us to understand the molecular mechanisms that enable eukaryotic cells to control the quality of mitochondrial proteins as they are being imported into the organelle and thus externally control the quality of the organelle. As impairment of mitochondrial function is a hallmark of many common human diseases, these results may also bear relevance to human health.

• DFG Programme: Research Grants
• Ehemaliger Antragsteller: Professor Dr. Walter Neupert, until 8/2019 (†)