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. 2013 Jun 6;50(5):637-48.
doi: 10.1016/j.molcel.2013.04.015. Epub 2013 May 16.

Listerin-dependent nascent protein ubiquitination relies on ribosome subunit dissociation

Sichen Shao  1 , Karina von der Malsburg, Ramanujan S Hegde
Affiliations

Listerin-dependent nascent protein ubiquitination relies on ribosome subunit dissociation

Sichen Shao et al. Mol Cell. .

Abstract

Quality control of defective mRNAs relies on their translation to detect the lesion. Aberrant proteins are therefore an obligate byproduct of mRNA surveillance and must be degraded to avoid disrupting protein homeostasis. These defective translation products are thought to be ubiquitinated at the ribosome, but the mechanism of ubiquitin ligase selectivity for these ribosomes is not clear. Here, we in vitro reconstitute ubiquitination of nascent proteins produced from aberrant mRNAs. Stalled 80S ribosome-nascent chain complexes are dissociated by the ribosome recycling factors Hbs1/Pelota/ABCE1 to a unique 60S-nascent chain-tRNA complex. The ubiquitin ligase Listerin preferentially recognizes 60S-nascent chains and triggers efficient nascent chain ubiquitination. Interfering with Hbs1 function stabilizes 80S complexes, precludes efficient Listerin recruitment, and reduces nascent chain ubiquitination. Thus, ribosome recycling factors control Listerin localization, explaining how translation products of mRNA surveillance are efficiently ubiquitinated while sparing translating ribosomes.

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Figures

Figure 1
Figure 1
Nascent Chain-tRNAs on Stalled Ribosomes Are Ubiquitinated In Vitro (A) Line diagram of truncated and full-length (FL) β-VHP transcripts. (B) Schematic diagram of a stalled β-VHP ribosome-nascent chain complex (RNC). (C) In vitro translation reactions of β-VHP RNCs containing FLAG-tagged ubiquitin were sequentially treated with RNase, precipitated with CTAB, and/or immunoprecipitated with anti-FLAG or control antibodies as indicated. The positions of the nascent chain (NC) containing or lacking a tRNA are indicated, along with ubiquitinated products (Ub). (D and E) Translation reactions of β-VHP or β-VHP-FL containing His-tagged ubiquitin were separated on 10%–50% sucrose gradients. Each fraction was immunoprecipitated with an antibody against the substrate and visualized by autoradiography. The upper portion of the gel was exposed 4-fold longer than the lower portion to visualize ubiquitinated products. An A260 trace is shown to indicate ribosomal fractions. An aliquot of pooled fractions 5–7 was also subject to immobilized Co2+ pull-downs to recover His-tagged ubiquitinated products (last lane). See also Figure S1.
Figure 2
Figure 2
Internally Stalled Ribosome-Nascent Chains Are Ubiquitinated (A) Line diagrams of transcripts coding for truncated β-VHP, truncated β-VHP with a poly(A) tail (β-VHP-pA), and full-length β-VHP with a polybasic tract (β-VHP-K12). (B) Truncated β-VHP with or without a poly(A) tail was translated and subject to RNase treatment. The migration of nascent chain-tRNA (NC-tRNA) and nascent chain (NC) is indicated. (C) Analysis of β-VHP-pA as in Figure 1C. (D) β-VHP-K12 was translated and subject to RNase treatment or immunoprecipitations with antibodies against either the N or C terminus of the protein. Immunoprecipitations of β-VHP-FL translations are shown for comparison. (E) Analysis of β-VHP-K12 as in Figure 1C. See also Figure S2.
Figure 3
Figure 3
Listerin Is Recruited to Stalled RNCs to Mediate Ubiquitination (A) β-VHP RNCs purified under physiological or high salt conditions (input) were incubated with the indicated components plus His-tagged ubiquitin and ATP, followed by pull-down of the ubiquitinated products via the His-tag (Ub-PD). (B) RNCs from (A) were stained for total protein or immunoblotted for Listerin (Ltn) and ribosomal protein S16. (C) S100 cytosol was immunodepleted with anti-Listerin (ΔLtn) or nonimmune (mock) antibody and stained for total proteins and immunoblotted for Listerin. (D) High salt-washed RNCs (input) were subjected to ubiquitination assays (as in A) with the indicated components and different relative amounts of the lysates from (C). (E) Translation reactions containing no mRNA or a truncated mRNA (β-VHP) were separated on 10%–50% sucrose gradients and individual fractions analyzed by immunoblotting for Listerin. The Listerin blot and its quantification are shown. (F) In vitro translation reactions of HA-tagged β-VHP RNCs (input) were immunoprecipitated with control or anti-HA antibodies in the presence of either 2 mM MgCl2 or 10 mM EDTA and analyzed by immunoblotting to detect Listerin (top) and the translation product (bottom). (G) HEK293T cells pretreated for 1 hr with 200 nM pactamycin, 50 μg/mL cycloheximide, or nothing were harvested and lysates fractionated by 10%–50% sucrose gradients. Fractions were analyzed for Listerin (quantified below the blots) and A260 absorbance. An aliquot of the total lysate was also included in the blots (last lane). See also Figure S3.
Figure 4
Figure 4
60S-Nascent Chain-tRNA Complex Is the Selective Target for Ubiquitination (A) β-VHP RNCs were produced in the presence of His-tagged ubiquitin and separated on a 10%–30% sucrose gradient. Individual fractions were analyzed for ubiquitinated products via His pull-down (Ub-PD), the nascent chain (total), Listerin, and ribosomal proteins of the 60S (L9) and 40S (S16) subunits. Fractions with 40S, 60S, and 80S complexes are shown. (B) HEK293T cells pretreated for 1 hr with 50 μg/mL cycloheximide were harvested and fractionated on a 10%–30% sucrose gradient and analyzed by immunoblotting for Listerin and ribosomal subunits. Peak fractions of key complexes are displayed. (C) β-VHP was translated in a ubiquitin- and E2-deficient fractionated translation extract (Fr-RRL) replenished with His-tagged ubiquitin (Ub) and 250 nM UbcH5a (E2) as indicated. The samples were analyzed directly (lower panel) or subject to His pull-downs of ubiquitinated products (top). (D) β-VHP RNCs produced in Fr-RRL were separated on a 10%–30% sucrose gradient and individual fractions incubated with or without E1 and E2 enzymes plus His-Ubiquitin and ATP. The samples were analyzed directly (total) and after ubiquitin pull-down (Ub-PD). The proportion of ubiquitinated substrate in each fraction was quantified (bottom). Blots of the fractions show the positions of Listerin and the ribosomal proteins. See also Figure S4.
Figure 5
Figure 5
Bipartite Recognition of 60S-Nascent Chain Substrates by Listerin (A) 33P-labeled β-VHP transcript was used to program a 60 min translation reaction, with 100 μM aurin tricarboxylic acid added at 10 min to inhibit initiation. Samples taken at 10 and 60 min were analyzed by gel electrophoresis and autoradiography (inset) or separated on a 10%–50% sucrose gradient and quantified by scintillation counting (graph). Ribosomes migrate in fractions 6–9. A matched reaction containing unlabeled transcript and 35S-methionine was analyzed in parallel for migration of the protein product (lower panel). (B) Scheme to generate 60S subunits containing or lacking a nascent chain after ribosome splitting. (C) Translation reactions of β-VHP (left) or ΔVHP-β (right) containing His-tagged ubiquitin were separated on a 10%–50% sucrose gradient. Individual fractions were analyzed by autoradiography (total) or immunoblotting for Listerin, or subjected to pull-downs for ubiquitinated products. See also Figure S5.
Figure 6
Figure 6
Ribosome Subunit Dissociation Precedes Listerin-Mediated Ubiquitination (A) Translation extracts were passed over a control (mock) or Pelota-conjugated resin (ΔHbs1) and blotted for Listerin and Hbs1 (left panel). β-VHP was translated in mock or ΔHbs1 lysates and analyzed directly (total) or subjected to ubiquitin pull-downs (Ub) before analysis. (B) ΔVHP-β was translated in the absence (control) or presence of 5-fold excess wild-type (WT) or dominant-negative (DN) H348A Hbs1 and separated on a 10%–50% gradient, and each fraction was analyzed by autoradiography. (C) Translation reactions of 33P-labeled β-VHP transcript in the absence (control) or presence of excess WT or DN Hbs1 were separated on a 10%–50% gradient, and each fraction was quantified by scintillation counting. (D) β-VHP transcripts lacking or containing an N-terminal 3× HA-tag were translated in the absence or presence of WT or two different dominant-negative Hbs1 mutants. The samples were analyzed directly (total) or after affinity purification via the HA tag (HA IPs). Translation products were analyzed by autoradiography, with a long exposure of the upper part of the gel to detect ubiquitinated species. Immunoblotting was used to detect copurifying Listerin and ribosomal proteins L9 and S16. (E) β-VHP was translated with an excess of WT or DN Hbs1, separated on a 10%–50% gradient, and the relative amount of Listerin in each fraction was quantified by blotting. See also Figure S6.
Figure 7
Figure 7
Model for Protein Quality Control Coupled to mRNA Surveillance The absence or skipping of a stop codon results in translation into the poly(A) tail and ribosome stalling. Listerin has poor affinity for either elongating or stalled ribosomes. The latter are selectively targeted by ribosome recycling factors (Pelota, Hbs1, and ABCE1) to generate a 60S-nascent chain complex and 40S-mRNA complex. The 60S-nascent chain complex recruits Listerin to ubiquitinate the nascent chain, while the freed mRNA can be degraded by mRNA surveillance pathways. Additional components of the ribosome quality control complex (RQC; not depicted) may facilitate downstream steps of nascent chain extraction and degradation. Similar events are proposed to occur for stalls at the 3′ end of a message, at internal polybasic domains, or areas of mRNA secondary structure. See also Figure S7.
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