Ubiquilin 2: Shuttling Clients Out of Phase?
2018; Elsevier BV; Volume: 69; Issue: 6 Linguagem: Inglês
10.1016/j.molcel.2018.02.030
ISSN1097-4164
AutoresIpsita Subudhi, James Shorter,
Tópico(s)RNA Research and Splicing
ResumoUbiquilin 2 (UBQLN2) is an amyotrophic lateral sclerosis-linked molecular chaperone with a prion-like domain that directly engages ubiquitin to triage clients for proteasomal degradation. Dao et al., 2018Dao T.P. Kolaitis R.-M. Kim H.J. O'Donovan K. Martyniak B. Colicino E. Hehnly H. Taylor J.P. Castañeda C.A. Ubiquitin modulates liquid-liquid phase separation of UBQLN2 via disruption of multivalent interactions.Mol. Cell. 2018; 69 (this issue): 965-978Abstract Full Text Full Text PDF PubMed Scopus (157) Google Scholar now establish that UBQLN2 forms ubiquitin-labile liquids, which may enable UBQLN2 to specifically extract ubiquitylated clients from stress granules for degradation. Ubiquilin 2 (UBQLN2) is an amyotrophic lateral sclerosis-linked molecular chaperone with a prion-like domain that directly engages ubiquitin to triage clients for proteasomal degradation. Dao et al., 2018Dao T.P. Kolaitis R.-M. Kim H.J. O'Donovan K. Martyniak B. Colicino E. Hehnly H. Taylor J.P. Castañeda C.A. Ubiquitin modulates liquid-liquid phase separation of UBQLN2 via disruption of multivalent interactions.Mol. Cell. 2018; 69 (this issue): 965-978Abstract Full Text Full Text PDF PubMed Scopus (157) Google Scholar now establish that UBQLN2 forms ubiquitin-labile liquids, which may enable UBQLN2 to specifically extract ubiquitylated clients from stress granules for degradation. In response to stress, cells implement several damage-limitation programs, including the biogenesis of cytoplasmic stress granules (SGs) (Protter and Parker, 2016Protter D.S. Parker R. Principles and properties of stress granules.Trends Cell Biol. 2016; 26: 668-679Abstract Full Text Full Text PDF PubMed Scopus (758) Google Scholar). These dynamic membraneless organelles assemble upon inhibition of translation initiation to protect their protein and RNA constituents during deleterious periods. Once stress ebbs, SGs resolve to liberate their proteins and RNAs, which may resume their functions or be degraded. SGs assemble via liquid-liquid phase separation (LLPS) driven by multivalent RNA-RNA, RNA-protein, and protein-protein interactions, including multiple transient contacts between low-complexity, prion-like domains (PrLDs) (March et al., 2016March Z.M. King O.D. Shorter J. Prion-like domains as epigenetic regulators, scaffolds for subcellular organization, and drivers of neurodegenerative disease.Brain Res. 2016; 1647: 9-18Crossref PubMed Scopus (143) Google Scholar, Protter and Parker, 2016Protter D.S. Parker R. Principles and properties of stress granules.Trends Cell Biol. 2016; 26: 668-679Abstract Full Text Full Text PDF PubMed Scopus (758) Google Scholar). Excessive SG assembly or defective SG dissolution after stress is linked with neurodegeneration and may reflect altered phase behavior of SG-resident RNA-binding proteins (RBPs) or altered protein-degradation programs (March et al., 2016March Z.M. King O.D. Shorter J. Prion-like domains as epigenetic regulators, scaffolds for subcellular organization, and drivers of neurodegenerative disease.Brain Res. 2016; 1647: 9-18Crossref PubMed Scopus (143) Google Scholar). Ubiquilins (UBQLNs) are a family of four (-1, -2, -3, and -4) paralogous molecular chaperones that shuttle ubiquitylated clients to the proteasome for degradation (Hjerpe et al., 2016Hjerpe R. Bett J.S. Keuss M.J. Solovyova A. McWilliams T.G. Johnson C. Sahu I. Varghese J. Wood N. Wightman M. et al.UBQLN2 mediates autophagy-independent protein aggregate clearance by the proteasome.Cell. 2016; 166: 935-949Abstract Full Text Full Text PDF PubMed Scopus (193) Google Scholar, Itakura et al., 2016Itakura E. Zavodszky E. Shao S. Wohlever M.L. Keenan R.J. Hegde R.S. Ubiquilins chaperone and triage mitochondrial membrane proteins for degradation.Mol. Cell. 2016; 63: 21-33Abstract Full Text Full Text PDF PubMed Scopus (144) Google Scholar). Mutations in UBQLN2 or UBQLN4 that impair protein degradation cause amyotrophic lateral sclerosis (ALS) (Deng et al., 2011Deng H.X. Chen W. Hong S.T. Boycott K.M. Gorrie G.H. Siddique N. Yang Y. Fecto F. Shi Y. Zhai H. et al.Mutations in UBQLN2 cause dominant X-linked juvenile and adult-onset ALS and ALS/dementia.Nature. 2011; 477: 211-215Crossref PubMed Scopus (876) Google Scholar, Edens et al., 2017Edens B.M. Yan J. Miller N. Deng H.X. Siddique T. Ma Y.C. A novel ALS-associated variant in UBQLN4 regulates motor axon morphogenesis.eLife. 2017; 6: e25453Crossref PubMed Scopus (32) Google Scholar). However, the mechanisms underlying the complete functional repertoire of UBQLNs are not understood. In this issue of Molecular Cell, Castañeda and coworkers determine that UBQLN2 undergoes LLPS and accumulates in SGs (Dao et al., 2018Dao T.P. Kolaitis R.-M. Kim H.J. O'Donovan K. Martyniak B. Colicino E. Hehnly H. Taylor J.P. Castañeda C.A. Ubiquitin modulates liquid-liquid phase separation of UBQLN2 via disruption of multivalent interactions.Mol. Cell. 2018; 69 (this issue): 965-978Abstract Full Text Full Text PDF PubMed Scopus (157) Google Scholar). Ubiquitin or polyubiquitin abolishes UBQLN2 LLPS and may enable UBQLN2 to extract ubiquitylated clients from SGs for degradation (Dao et al., 2018Dao T.P. Kolaitis R.-M. Kim H.J. O'Donovan K. Martyniak B. Colicino E. Hehnly H. Taylor J.P. Castañeda C.A. Ubiquitin modulates liquid-liquid phase separation of UBQLN2 via disruption of multivalent interactions.Mol. Cell. 2018; 69 (this issue): 965-978Abstract Full Text Full Text PDF PubMed Scopus (157) Google Scholar). Dao et al. establish that UBQLN2 is diffusely localized in the cytoplasm, but accumulates in SGs under a variety of stresses (Dao et al., 2018Dao T.P. Kolaitis R.-M. Kim H.J. O'Donovan K. Martyniak B. Colicino E. Hehnly H. Taylor J.P. Castañeda C.A. Ubiquitin modulates liquid-liquid phase separation of UBQLN2 via disruption of multivalent interactions.Mol. Cell. 2018; 69 (this issue): 965-978Abstract Full Text Full Text PDF PubMed Scopus (157) Google Scholar). Importantly, purified UBQLN2 undergoes reversible LLPS in vitro under relatively physiological conditions, which likely enables UBQLN2 recruitment to SGs in vivo (Dao et al., 2018Dao T.P. Kolaitis R.-M. Kim H.J. O'Donovan K. Martyniak B. Colicino E. Hehnly H. Taylor J.P. Castañeda C.A. Ubiquitin modulates liquid-liquid phase separation of UBQLN2 via disruption of multivalent interactions.Mol. Cell. 2018; 69 (this issue): 965-978Abstract Full Text Full Text PDF PubMed Scopus (157) Google Scholar). To map the domain requirements for UBQLN2 LLPS, several UBQLN2 deletion constructs were assessed for propensity to form liquid drops. Like other UBQLNs, UBQLN2 harbors a ubiquitin-like domain (UBL; residues 33–103) that associates with proteasomal subunits, two STI1 domains (STI1-I, residues 178–247, and STI1-II, residues 379–462) that interact with molecular chaperones including Hsp70, and a ubiquitin-associating domain (UBA, residues 577–620) that binds to ubiquitin (Figure 1A) (Ko et al., 2004Ko H.S. Uehara T. Tsuruma K. Nomura Y. Ubiquilin interacts with ubiquitylated proteins and proteasome through its ubiquitin-associated and ubiquitin-like domains.FEBS Lett. 2004; 566: 110-114Crossref PubMed Scopus (140) Google Scholar). UBQLN2 also contains a proline-rich region (Pxx) (residues 491–538), which is not found in other UBQLNs. Like UBQLN1, UBQLN2 harbors a PrLD (residues 339–462), which overlaps with STI1-II (Figure 1A) (March et al., 2016March Z.M. King O.D. Shorter J. Prion-like domains as epigenetic regulators, scaffolds for subcellular organization, and drivers of neurodegenerative disease.Brain Res. 2016; 1647: 9-18Crossref PubMed Scopus (143) Google Scholar). PrLDs are low-complexity domains with a distinctive amino acid composition enriched in glycine and uncharged polar residues such as glutamine, asparagine, tyrosine, and serine, which resemble prion domains that enable various yeast proteins to form prions (March et al., 2016March Z.M. King O.D. Shorter J. Prion-like domains as epigenetic regulators, scaffolds for subcellular organization, and drivers of neurodegenerative disease.Brain Res. 2016; 1647: 9-18Crossref PubMed Scopus (143) Google Scholar). PrLDs also drive functional LLPS and deleterious aggregation of several ALS-linked RBPs such as TDP-43 and FUS (Figure 1A) (March et al., 2016March Z.M. King O.D. Shorter J. Prion-like domains as epigenetic regulators, scaffolds for subcellular organization, and drivers of neurodegenerative disease.Brain Res. 2016; 1647: 9-18Crossref PubMed Scopus (143) Google Scholar). Dao et al. show that UBQLN2 450-624, which includes a portion of the PrLD, STI1-II, Pxx, and UBA domains, is the minimal region necessary for LLPS in vitro and accumulation in SGs in vivo (Dao et al., 2018Dao T.P. Kolaitis R.-M. Kim H.J. O'Donovan K. Martyniak B. Colicino E. Hehnly H. Taylor J.P. Castañeda C.A. Ubiquitin modulates liquid-liquid phase separation of UBQLN2 via disruption of multivalent interactions.Mol. Cell. 2018; 69 (this issue): 965-978Abstract Full Text Full Text PDF PubMed Scopus (157) Google Scholar). NMR revealed this region to be largely disordered. Indeed, the UBA domain is folded, but residues 487–580 are intrinsically disordered with backbone relaxation parameters akin to the FUS PrLD (Dao et al., 2018Dao T.P. Kolaitis R.-M. Kim H.J. O'Donovan K. Martyniak B. Colicino E. Hehnly H. Taylor J.P. Castañeda C.A. Ubiquitin modulates liquid-liquid phase separation of UBQLN2 via disruption of multivalent interactions.Mol. Cell. 2018; 69 (this issue): 965-978Abstract Full Text Full Text PDF PubMed Scopus (157) Google Scholar). However, robust UBQLN2 LLPS under diverse conditions requires the STI1-II portion of the PrLD (Dao et al., 2018Dao T.P. Kolaitis R.-M. Kim H.J. O'Donovan K. Martyniak B. Colicino E. Hehnly H. Taylor J.P. Castañeda C.A. Ubiquitin modulates liquid-liquid phase separation of UBQLN2 via disruption of multivalent interactions.Mol. Cell. 2018; 69 (this issue): 965-978Abstract Full Text Full Text PDF PubMed Scopus (157) Google Scholar). Deletion of the STI1-II portion of the PrLD or the UBA domain abolishes UBQLN2 LLPS in vitro (Dao et al., 2018Dao T.P. Kolaitis R.-M. Kim H.J. O'Donovan K. Martyniak B. Colicino E. Hehnly H. Taylor J.P. Castañeda C.A. Ubiquitin modulates liquid-liquid phase separation of UBQLN2 via disruption of multivalent interactions.Mol. Cell. 2018; 69 (this issue): 965-978Abstract Full Text Full Text PDF PubMed Scopus (157) Google Scholar). Thus, several UBQLN2 domains engage in multivalent interactions that elicit LLPS, including major contributions from the STI1-II portion of the PrLD and the UBA domain. To shuttle ubiquitylated clients to the proteasome, UBQLNs bind ubiquitin and proteasomes directly (Itakura et al., 2016Itakura E. Zavodszky E. Shao S. Wohlever M.L. Keenan R.J. Hegde R.S. Ubiquilins chaperone and triage mitochondrial membrane proteins for degradation.Mol. Cell. 2016; 63: 21-33Abstract Full Text Full Text PDF PubMed Scopus (144) Google Scholar, Ko et al., 2004Ko H.S. Uehara T. Tsuruma K. Nomura Y. Ubiquilin interacts with ubiquitylated proteins and proteasome through its ubiquitin-associated and ubiquitin-like domains.FEBS Lett. 2004; 566: 110-114Crossref PubMed Scopus (140) Google Scholar). Remarkably, UBQLN2 binding to ubiquitin or polyubiquitin in vitro disrupted UBQLN2 LLPS. Indeed, UBQLN2 LLPS was eliminated by monoubiquitin and K48-linked di- and tetra-ubiquitin chains, a ubiquitin linkage associated with proteasomal degradation. Specific binding of ubiquitin to the UBA domain disrupts multivalent interactions that maintain UBQLN2 liquids (Dao et al., 2018Dao T.P. Kolaitis R.-M. Kim H.J. O'Donovan K. Martyniak B. Colicino E. Hehnly H. Taylor J.P. Castañeda C.A. Ubiquitin modulates liquid-liquid phase separation of UBQLN2 via disruption of multivalent interactions.Mol. Cell. 2018; 69 (this issue): 965-978Abstract Full Text Full Text PDF PubMed Scopus (157) Google Scholar). Thus, the UBA domain acts as a regulatory switch for UBQLN2 LLPS. As many proteins are ubiquitylated in SGs, UBQLN2 may bind to these ubiquitin chains inside the SG and subsequently disassociate from the SG phase to ferry substrates to the proteasome (Figure 1B) (Dao et al., 2018Dao T.P. Kolaitis R.-M. Kim H.J. O'Donovan K. Martyniak B. Colicino E. Hehnly H. Taylor J.P. Castañeda C.A. Ubiquitin modulates liquid-liquid phase separation of UBQLN2 via disruption of multivalent interactions.Mol. Cell. 2018; 69 (this issue): 965-978Abstract Full Text Full Text PDF PubMed Scopus (157) Google Scholar). While Dao et al. focused on K48-linked ubiquitin chains, it will also be interesting to consider other ubiquitin linkages. Given that UBQLNs are also connected to macroautophagy and interact with LC3, an autophagosome marker (Rothenberg et al., 2010Rothenberg C. Srinivasan D. Mah L. Kaushik S. Peterhoff C.M. Ugolino J. Fang S. Cuervo A.M. Nixon R.A. Monteiro M.J. Ubiquilin functions in autophagy and is degraded by chaperone-mediated autophagy.Hum. Mol. Genet. 2010; 19: 3219-3232Crossref PubMed Scopus (183) Google Scholar), perhaps UBQLN2 operates in a similar manner with alternative ubiquitin linkages to target clients for autophagy. Dao et al. advance a model in which UBQLN2 undergoes LLPS driven by multivalent interactions mediated by the STI1-II portion of the PrLD, the Pxx domain, and the UBA domain, which enables SG association in vivo. Upon binding to ubiquitin chains via its UBA domain, UBQLN2 LLPS is disrupted, which elicits release of client-bound UBQLN2 from the SG to enable client degradation by the proteasome (Figure 1B). This model provides a mechanism by which UBQLN2 may extract ubiquitylated, ALS-linked RBPs such as TDP-43 and FUS from SGs (Figure 1B) (Dao et al., 2018Dao T.P. Kolaitis R.-M. Kim H.J. O'Donovan K. Martyniak B. Colicino E. Hehnly H. Taylor J.P. Castañeda C.A. Ubiquitin modulates liquid-liquid phase separation of UBQLN2 via disruption of multivalent interactions.Mol. Cell. 2018; 69 (this issue): 965-978Abstract Full Text Full Text PDF PubMed Scopus (157) Google Scholar). Moreover, interactions between the UBQLN2 PrLD and PrLDs of TDP-43 and FUS may fluidize SGs and prevent aberrant phase transitions to pathological aggregates (March et al., 2016March Z.M. King O.D. Shorter J. Prion-like domains as epigenetic regulators, scaffolds for subcellular organization, and drivers of neurodegenerative disease.Brain Res. 2016; 1647: 9-18Crossref PubMed Scopus (143) Google Scholar). These proposed UBQLN2 activities could have therapeutic utility and are likely corrupted in disease. Indeed, UBQLN2 co-localizes with cytoplasmic inclusions in a wide variety of ALS cases (Deng et al., 2011Deng H.X. Chen W. Hong S.T. Boycott K.M. Gorrie G.H. Siddique N. Yang Y. Fecto F. Shi Y. Zhai H. et al.Mutations in UBQLN2 cause dominant X-linked juvenile and adult-onset ALS and ALS/dementia.Nature. 2011; 477: 211-215Crossref PubMed Scopus (876) Google Scholar). However, these inclusions could be resolved with therapeutic protein disaggregases (Shorter, 2017Shorter J. Designer protein disaggregases to counter neurodegenerative disease.Curr. Opin. Genet. Dev. 2017; 44: 1-8Crossref PubMed Scopus (47) Google Scholar). Moving forward, it will be critical to confirm whether UBQLN2 engages ubiquitylated clients inside SGs and removes them from the SG phase for proteasomal degradation in vivo. It will also be important to determine the LLPS propensity of ALS-linked UBQLN2 mutations, the majority of which cluster near the proline-rich Pxx region (Deng et al., 2011Deng H.X. Chen W. Hong S.T. Boycott K.M. Gorrie G.H. Siddique N. Yang Y. Fecto F. Shi Y. Zhai H. et al.Mutations in UBQLN2 cause dominant X-linked juvenile and adult-onset ALS and ALS/dementia.Nature. 2011; 477: 211-215Crossref PubMed Scopus (876) Google Scholar). Finally, it will be enlightening to learn whether ALS-linked UBQLN2 variants are defective in ubiquitylated client extraction from SGs. Ubiquitin Modulates Liquid-Liquid Phase Separation of UBQLN2 via Disruption of Multivalent InteractionsDao et al.Molecular CellMarch 8, 2018In BriefUbiquilins (UBQLNs) are shuttle proteins essential for cellular protein quality control machinery. Dao et al. show that UBQLN2 colocalizes with stress granules in vivo and undergoes liquid-liquid phase separation at physiological conditions in vitro. Ubiquitin binding induces a transition that reverses UBQLN2 phase separation. Full-Text PDF Open Archive
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