The Structure of Ribosome-Channel Complexes Engaged in Protein Translocation

Department of Physiology and Biophysics Boston University School of Medicine, Boston, MA 02118, USA.
Molecular Cell (Impact Factor: 14.02). 12/2000; 6(5):1219-32. DOI: 10.1016/S1097-2765(00)00118-0
Source: PubMed


Cotranslational translocation of proteins requires ribosome binding to the Sec61p channel at the endoplasmic reticulum (ER) membrane. We have used electron cryomicroscopy to determine the structures of ribosome-channel complexes in the absence or presence of translocating polypeptide chains. Surprisingly, the structures are similar and contain 3-4 connections between the ribosome and channel that leave a lateral opening into the cytosol. Therefore, the ribosome-channel junction may allow the direct transfer of polypeptides into the channel and provide a path for the egress of some nascent chains into the cytosol. Moreover, complexes solubilized from mammalian ER membranes contain an additional membrane protein that has a large, lumenal protrusion and is intercalated into the wall of the Sec61p channel. Thus, the native channel contains a component that is not essential for translocation.

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    • "– translocon junction ( Jungnickel and Rapoport , 1995 ) . This is supported by protease accessibility assays in this and earlier studies ( e . g . , Jungnickel and Rapoport , 1995 ; Kim et al . , 2002 ) . Such a gap has long been observed structurally and postulated to facilitate egress of cytosolic domains during mem - brane protein biogenesis ( Menetret et al . , 2000 ; van den Berg et al . , 2004 ) . Indeed , the lateral gate is positioned such that cytosolic do - mains adjacent to transmembrane segments access this gap during membrane insertion into the lipid bilayer ( Gogala et al . , 2014 ) . Thus , although not directly examined in this study , stalled integral mem - brane proteins should typica"
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    ABSTRACT: Cytosolic ribosomes that stall during translation are split into subunits, and nascent polypeptides trapped in the 60S subunit are ubiquitinated by the ribosome quality control (RQC) pathway. Whether the RQC pathway can also target stalls during cotranslational translocation into the ER is not known. Here, we report that Listerin and NEMF, core RQC components, are bound to translocon-engaged 60S subunits on native ER membranes. RQC recruitment to the ER in cultured cells is stimulated by translation stalling. Biochemical analyses demonstrated that translocon-targeted nascent polypeptides that subsequently stall are polyubiquitinated in 60S complexes. Ubiquitination at the translocon requires cytosolic exposure of the polypeptide at the ribosome-Sec61 junction. This exposure can result from either failed insertion into the Sec61 channel or partial backsliding of translocating nascent chains. Only Sec61-engaged nascent chains early in their biogenesis were relatively refractory to ubiquitination. Modelling based on recent 60S-RQC and 80S-Sec61 structures suggests that the E3 ligase Listerin accesses nascent polypeptides via a gap in the ribosome-translocon junction near the Sec61 lateral gate. Thus, the RQC pathway can target stalled translocation intermediates for degradation from the Sec61 channel. © 2015 by The American Society for Cell Biology.
    Molecular biology of the cell 04/2015; 26(12). DOI:10.1091/mbc.E15-01-0040 · 4.47 Impact Factor
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    • "The subsequent stages of cotranslational translocation also remain to be resolved mechanistically. The various ribosome- PCC structures show that protein translocation is not accompanied by any major structural changes to the PCC (Menetret et al., 2000; Gogala et al., 2014). By contrast, engagement of a signal peptide or transmembrane domain opens the lateral gate to varying degrees (Park et al., 2014; Gogala et al., 2014), which may result in a conformation similar to that observed when a symmetry-related protein partially parted the lateral gate of SecY (Egea and Stroud, 2010). "
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    • "To analyze the fate of newly synthesized cytochrome b in the absence of Cbp3, we followed the synthesis and stability of mitochondrially encoded proteins in whole cells (Fig. 2 B). To avoid any interference with the complex splicing of mitochondrial peptide deformylase, (2) chaperones like trigger factor, and (3) targeting and membrane insertion components like the signal recognition particle and the SecYEG complex (Ménétret et al., 2000; Kramer et al., 2002; Gu et al., 2003; Bingel-Erlenmeyer et al., 2008). These factors interact with the rim of the polypeptide tunnel exit that is formed by RNA moieties and four conserved proteins, namely L22, L23, L24, and L29. "
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    ABSTRACT: Mitochondria contain their own genetic system to express a small number of hydrophobic polypeptides, including cytochrome b, an essential subunit of the bc(1) complex of the respiratory chain. In this paper, we show in yeast that Cbp3, a bc(1) complex assembly factor, and Cbp6, a regulator of cytochrome b translation, form a complex that associates with the polypeptide tunnel exit of mitochondrial ribosomes and that exhibits two important functions in the biogenesis of cytochrome b. On the one hand, the interaction of Cbp3 and Cbp6 with mitochondrial ribosomes is necessary for efficient translation of cytochrome b transcript [corrected]. On the other hand, the Cbp3-Cbp6 complex interacts directly with newly synthesized cytochrome b in an assembly intermediate that is not ribosome bound and that contains the assembly factor Cbp4. Our results suggest that synthesis of cytochrome b occurs preferentially on those ribosomes that have the Cbp3-Cbp6 complex bound to their tunnel exit, an arrangement that may ensure tight coordination of cytochrome b synthesis and assembly.
    The Journal of Cell Biology 06/2011; 193(6):1101-14. DOI:10.1083/jcb.201103132 · 9.83 Impact Factor
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