Elucidating how cells dispose of aberrant proteins

Understanding disease and discovering therapeutics

Overview

We study how cells control the quality of their proteins. More specifically, we aim to understand the mechanisms involved in sensing and eliminating aberrant or damaged proteins. Such mechanisms are critical to ensure the fidelity of gene expression, the integrity of the proteome, and cellular fitness. In humans, defects in protein quality control resulting in the accumulation and aggregation of aberrant proteins are a hallmark of neurodegenerative diseases.

 

Our approaches rely on identifying and functionally characterizing factors that confer specificity to protein quality control, such as E3 ubiquitin ligases, ribosome collision sensors, and the ribosome- and tRNA-binding proteins of the RqcH/Rqc2/NEMF family. Furthermore, we leverage our findings towards developing new small-molecule research tools and therapeutics.

Ribosome-associated Quality Control (RQC): mechanisms, evolution, role in ageing, and defects causing neurodegeneration

For the past several years, we have focused on understanding protein quality control during translation. Protein synthesis does not always go as planned. For example, a ribosome can stall while making a new protein for a variety of reasons, such mRNA damage or amino acid scarcity. The incomplete nascent polypeptide produced by ribosome stalling is potentially toxic and must be eliminated. We are elucidating how that happens. When a ribosome stalls, a trailing ribosome can collide with it. We have found that ribosome collisions are a deeply-conserved signal across evolution for triggering responses that minimize damage caused by interrupted translation. One such response is Ribosome-associated Quality Control (RQC), which marks the incomplete nascent polypeptide for degradation.

We study RQC across evolution and across scales, from solving near-atomic structures of the bacterial RQC machinery to generating mouse models of RQC dysfunction. We wish to understand the mechanisms underlying RQC, the principles and logic of the process, and its roles in biology and disease. In addition to our findings linking RQC dysfunction to neuromuscular phenotypes in mice and humans, a promising new research direction is that RQC is emerging as the next frontier in ageing research.                          joaziero  joazierolab   joazierolab.com

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We study protein quality control from several angles
  • Molecular mechanisms

  • Structures (through collaborations)

  • Evolution (bacteria, yeast, mice, human cells)

  • Disease relevance

  • Small molecule applications

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Cast of molecular characters
  • Stalled and collided ribosomes

  • Obstructed large ribosomal subunit

  • Rescue factors: RqcU ATPase

  • Ala tailing factors: Rqc2/RqcH/NEMF and RqcP

  • E3 ligases: Ltn1, Pirh2, Klhdc10

  • ubiquitin

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Diverse model systems and approaches
  • Model systems: B. subtilis, S. cerevisiae, mammalian cell culture, human iPS cells and motor neurons, mice

  • Approaches: molecular genetics, biochemistry

  • Collaborations: cryo-EM, chemical biology

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Diseases we study
  • Cancer (RQC, PROTACs)

  • Virology (SARS-CoVs PROTACs)

  • Neurodegeneration (RQC, small molecules)

  • Ageing-related diseases (RQC)

How we got here

Discovery of the RING domain E3 ubiquitin ligase family

During post-doctoral work, while investigating how the duration and amplitude of kinase signaling is kept in check, we showed that the negative regulator of receptor tyrosine kinases, c-Cbl, acts as an E3 ligase (Joazeiro et al. 1999). We found that c-Cbl recognizes activated receptors through its variant SH2 domain and recruits E2 ubiquitin conjugating enzymes via its RING domain; c-Cbl then stimulates ubiquitin transfer from the E2 to receptors, to promote their degradation. Following this breakthrough discovery, we provided evidence, together with other labs, that RING domain proteins indeed have a general function as E3 ligases, greatly expanding the number of known and potential E3s (see Publication List).

The human genome encodes over 650 E3 ligases

It had long been anticipated that many E3 ligases must exist in order to account for specificity on the ubiquitylation of thousands of different proteins. To address this issue, our team set out to generate a comprehensive, genome-wide inventory of predicted human E3 ligases. The analyses identified and mapped over 650 E3-encoding genes, ~95% of which depend on a RING domain (Li et al. 2008; Deshaies & Joazeiro 2009). This inventory has enabled generating tools for functional genomic analyses focused on the E3 ligase family. As many E3 ligases play critical roles in biology and are directly implicated in disease (e.g., Parkin, Mdm2, BRCA1, to name a few), research into this family will continue to be a rich source of discoveries.

Mutation in the E3 ligase Ltn1/Listerin causes motor neurodegeneration in mice

We have reported that mice with a mutation in a previously unstudied gene, encoding the Listerin/LTN1 E3 ligase, develop neurologic and motor dysfunction similar to mouse models of Amyotrophic Lateral Sclerosis (ALS) (Chu et al 2009). ALS, a severely debilitating disease, took the lives of the American baseball player, Lou Gehrig, and the British physicist, Stephen Hawking. ALS gained further notoriety more recently, due to the ‘ice-bucket competition’ that highlighted the need for more research towards understanding and treating the disease. Genetic mouse models, like the Listerin-mutant mice, can provide insight into the pathogenesis and molecular pathways underlying ALS and other neurodegenerative diseases.

Discovery of Ribosome-associated Quality Control

The finding that Listerin mutation causes neurodegeneration in mice raised the questions of what Listerin’s normal function is, and how a defect in this function results in the phenotype. Our curiosity to know the answers to these questions led to the discovery that Listerin mediates Ribosome-associated Quality Control (RQC), a novel protein quality control pathway (Bengtson & Joazeiro 2010). Importantly, this finding provided the first link between RQC and neurodegeneration, which is consistent with the knowledge that defective protein quality control is a hallmark of human neurodegenerative diseases. See the RESEARCH tab to learn about how we have continued to shape the RQC field, as well as future directions we plan to take.

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ZMBH - Center for Molecular Biology 

Universität Heidelberg

Im Neuenheimer Feld 345, 1.OG

D-69120 Heidelberg, Germany

+49-6221-54-6846

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The JoazeiroLab appreciates the support from:

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