Defining E3 ligase-substrate relationships through multiplex CRISPR screening.

Acknowledgements: We thank C. Araneo and his team for FACS. R.T.T. is a Sir Henry Wellcome Postdoctoral Fellow (201387/Z/16/Z) and a Pemberton-Trinity Fellow. E.L.M. is an HHMI Fellow of The Jane Coffin Childs Memorial Fund for Medical Research. I.A.T. is a Damon Runyon-Dale F. Frey Breakthrough Scientist supported (in part) by the Damon Runyon Cancer Research Foundation (DFS-2277-16). I.K. is supported by the European Research Council (ERC-2020-STG 947709), the Israel Science Foundation (2380/21 and 3096/21), an Alon Fellowship and The Applebaum Foundation. This work was supported by an NIH grant AG11085 to S.J.E. S.J.E. is an Investigator with the Howard Hughes Medical Institute.Specificity within the ubiquitin-proteasome system is primarily achieved through E3 ubiquitin ligases, but for many E3s their substrates-and in particular the molecular features (degrons) that they recognize-remain largely unknown. Current approaches for assigning E3s to their cognate substrates are tedious and low throughput. Here we developed a multiplex CRISPR screening platform to assign E3 ligases to their cognate substrates at scale. A proof-of-principle multiplex screen successfully performed ~100 CRISPR screens in a single experiment, refining known C-degron pathways and identifying an additional pathway through which Cul2FEM1B targets C-terminal proline. Further, by identifying substrates for Cul1FBXO38, Cul2APPBP2, Cul3GAN, Cul3KLHL8, Cul3KLHL9/13 and Cul3KLHL15, we demonstrate that the approach is compatible with pools of full-length protein substrates of varying stabilities and, when combined with site-saturation mutagenesis, can assign E3 ligases to their cognate degron motifs. Thus, multiplex CRISPR screening will accelerate our understanding of how specificity is achieved within the ubiquitin-proteasome system

Defining E3 ligase–substrate relationships through multiplex CRISPR screening

Acknowledgements: We thank C. Araneo and his team for FACS. R.T.T. is a Sir Henry Wellcome Postdoctoral Fellow (201387/Z/16/Z) and a Pemberton-Trinity Fellow. E.L.M. is an HHMI Fellow of The Jane Coffin Childs Memorial Fund for Medical Research. I.A.T. is a Damon Runyon-Dale F. Frey Breakthrough Scientist supported (in part) by the Damon Runyon Cancer Research Foundation (DFS-2277-16). I.K. is supported by the European Research Council (ERC-2020-STG 947709), the Israel Science Foundation (2380/21 and 3096/21), an Alon Fellowship and The Applebaum Foundation. This work was supported by an NIH grant AG11085 to S.J.E. S.J.E. is an Investigator with the Howard Hughes Medical Institute.Specificity within the ubiquitin–proteasome system is primarily achieved through E3 ubiquitin ligases, but for many E3s their substrates—and in particular the molecular features (degrons) that they recognize—remain largely unknown. Current approaches for assigning E3s to their cognate substrates are tedious and low throughput. Here we developed a multiplex CRISPR screening platform to assign E3 ligases to their cognate substrates at scale. A proof-of-principle multiplex screen successfully performed ~100 CRISPR screens in a single experiment, refining known C-degron pathways and identifying an additional pathway through which Cul2FEM1B targets C-terminal proline. Further, by identifying substrates for Cul1FBXO38, Cul2APPBP2, Cul3GAN, Cul3KLHL8, Cul3KLHL9/13 and Cul3KLHL15, we demonstrate that the approach is compatible with pools of full-length protein substrates of varying stabilities and, when combined with site-saturation mutagenesis, can assign E3 ligases to their cognate degron motifs. Thus, multiplex CRISPR screening will accelerate our understanding of how specificity is achieved within the ubiquitin–proteasome system

Structural basis for dimerization quality control.

Most quality control pathways target misfolded proteins to prevent toxic aggregation and neurodegeneration1. Dimerization quality control further improves proteostasis by eliminating complexes of aberrant composition2, but how it detects incorrect subunits remains unknown. Here we provide structural insight into target selection by SCF-FBXL17, a dimerization-quality-control E3 ligase that ubiquitylates and helps to degrade inactive heterodimers of BTB proteins while sparing functional homodimers. We find that SCF-FBXL17 disrupts aberrant BTB dimers that fail to stabilize an intermolecular β-sheet around a highly divergent β-strand of the BTB domain. Complex dissociation allows SCF-FBXL17 to wrap around a single BTB domain, resulting in robust ubiquitylation. SCF-FBXL17 therefore probes both shape and complementarity of BTB domains, a mechanism that is well suited to establish quality control of complex composition for recurrent interaction modules

Structural basis and regulation of the reductive stress response.

Although oxidative phosphorylation is best known for producing ATP, it also yields reactive oxygen species (ROS) as invariant byproducts. Depletion of ROS below their physiological levels, a phenomenon known as reductive stress, impedes cellular signaling and has been linked to cancer, diabetes, and cardiomyopathy. Cells alleviate reductive stress by ubiquitylating and degrading the mitochondrial gatekeeper FNIP1, yet it is unknown how the responsible E3 ligase CUL2FEM1B can bind its target based on redox state and how this is adjusted to changing cellular environments. Here, we show that CUL2FEM1B relies on zinc as a molecular glue to selectively recruit reduced FNIP1 during reductive stress. FNIP1 ubiquitylation is gated by pseudosubstrate inhibitors of the BEX family, which prevent premature FNIP1 degradation to protect cells from unwarranted ROS accumulation. FEM1B gain-of-function mutation and BEX deletion elicit similar developmental syndromes, showing that the zinc-dependent reductive stress response must be tightly regulated to maintain cellular and organismal homeostasis

Systematic characterization of mutations altering protein degradation in human cancers.

The ubiquitin-proteasome system (UPS) is the primary route for selective protein degradation in human cells. The UPS is an attractive target for novel cancer therapies, but the precise UPS genes and substrates important for cancer growth are incompletely understood. Leveraging multi-omics data across more than 9,000 human tumors and 33 cancer types, we found that over 19% of all cancer driver genes affect UPS function. We implicate transcription factors as important substrates and show that c-Myc stability is modulated by CUL3. Moreover, we developed a deep learning model (deepDegron) to identify mutations that result in degron loss and experimentally validated the prediction that gain-of-function truncating mutations in GATA3 and PPM1D result in increased protein stability. Last, we identified UPS driver genes associated with prognosis and the tumor microenvironment. This study demonstrates the important role of UPS dysregulation in human cancer and underscores the potential therapeutic utility of targeting the UPS

Dynamic nuclear polarization at 700MHz/460GHz

We describe the design and implementation of the instrumentation required to perform DNP-NMR at higher field strengths than previously demonstrated, and report the first magic-angle spinning (MAS) DNP-NMR experiments performed at ¹H/e⁻ frequencies of 700 MHz/460 GHz. The extension of DNP-NMR to 16.4 T has required the development of probe technology, cryogenics, gyrotrons, and microwave transmission lines. The probe contains a 460 GHz microwave channel, with corrugated waveguide, tapers, and miter-bends that couple microwave power to the sample. Experimental efficiency is increased by a cryogenic exchange system for 3.2 mm rotors within the 89 mm bore. Sample temperatures ⩽85 K, resulting in improved DNP enhancements, are achieved by a novel heat exchanger design, stainless steel and brass vacuum jacketed transfer lines, and a bronze probe dewar. In addition, the heat exchanger is preceded with a nitrogen drying and generation system in series with a pre-cooling refrigerator. This reduces liquid nitrogen usage from >700 l per day to 7 days) cryogenic spinning without detrimental frost or ice formation. Initial enhancements, ε = −40, and a strong microwave power dependence suggests the possibility for considerable improvement. Finally, two-dimensional spectra of a model system demonstrate that the higher field provides excellent resolution, even in a glassy, cryoprotecting matrix.National Institutes of Health (U.S.) (EB002804)National Institutes of Health (U.S.) (EB003151)National Institutes of Health (U.S.) (EB002026)National Institutes of Health (U.S.) (EB001960)National Institutes of Health (U.S.) (EB001035)National Institutes of Health (U.S.) (EB001965)National Institutes of Health (U.S.) (EB004866