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      Defining the physiological role of SRP in protein-targeting efficiency and specificity

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      Science

      American Association for the Advancement of Science (AAAS)

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          Abstract

          The signal recognition particle (SRP) enables cotranslational delivery of proteins for translocation into the endoplasmic reticulum (ER), but its full in vivo role remains incompletely explored. We combined rapid auxin-induced SRP degradation with proximity-specific ribosome profiling to define SRP’s in vivo function in yeast. Despite the classic view that SRP recognizes N-terminal signal sequences, we show that SRP was generally essential for targeting transmembrane domains regardless of their position relative to the N terminus. By contrast, many proteins containing cleavable N-terminal signal peptides were efficiently cotranslationally targeted in SRP’s absence. We also revealed an unanticipated consequence of SRP loss: Normally ER-targeted transcripts were mistargeted to mitochondria, leading to mitochondrial defects. These results elucidate SRP’s essential roles in maintaining the efficiency and specificity of protein targeting.

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          Most cited references 31

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          A combined transmembrane topology and signal peptide prediction method.

          An inherent problem in transmembrane protein topology prediction and signal peptide prediction is the high similarity between the hydrophobic regions of a transmembrane helix and that of a signal peptide, leading to cross-reaction between the two types of predictions. To improve predictions further, it is therefore important to make a predictor that aims to discriminate between the two classes. In addition, topology information can be gained when successfully predicting a signal peptide leading a transmembrane protein since it dictates that the N terminus of the mature protein must be on the non-cytoplasmic side of the membrane. Here, we present Phobius, a combined transmembrane protein topology and signal peptide predictor. The predictor is based on a hidden Markov model (HMM) that models the different sequence regions of a signal peptide and the different regions of a transmembrane protein in a series of interconnected states. Training was done on a newly assembled and curated dataset. Compared to TMHMM and SignalP, errors coming from cross-prediction between transmembrane segments and signal peptides were reduced substantially by Phobius. False classifications of signal peptides were reduced from 26.1% to 3.9% and false classifications of transmembrane helices were reduced from 19.0% to 7.7%. Phobius was applied to the proteomes of Homo sapiens and Escherichia coli. Here we also noted a drastic reduction of false classifications compared to TMHMM/SignalP, suggesting that Phobius is well suited for whole-genome annotation of signal peptides and transmembrane regions. The method is available at as well as at
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            An auxin-based degron system for the rapid depletion of proteins in nonplant cells.

            Plants have evolved a unique system in which the plant hormone auxin directly induces rapid degradation of the AUX/IAA family of transcription repressors by a specific form of the SCF E3 ubiquitin ligase. Other eukaryotes lack the auxin response but share the SCF degradation pathway, allowing us to transplant the auxin-inducible degron (AID) system into nonplant cells and use a small molecule to conditionally control protein stability. The AID system allowed rapid and reversible degradation of target proteins in response to auxin and enabled us to generate efficient conditional mutants of essential proteins in yeast as well as cell lines derived from chicken, mouse, hamster, monkey and human cells, thus offering a powerful tool to control protein expression and study protein function.
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              An ER-mitochondria tethering complex revealed by a synthetic biology screen.

              Communication between organelles is an important feature of all eukaryotic cells. To uncover components involved in mitochondria/endoplasmic reticulum (ER) junctions, we screened for mutants that could be complemented by a synthetic protein designed to artificially tether the two organelles. We identified the Mmm1/Mdm10/Mdm12/Mdm34 complex as a molecular tether between ER and mitochondria. The tethering complex was composed of proteins resident of both ER and mitochondria. With the use of genome-wide mapping of genetic interactions, we showed that the components of the tethering complex were functionally connected to phospholipid biosynthesis and calcium-signaling genes. In mutant cells, phospholipid biosynthesis was impaired. The tethering complex localized to discrete foci, suggesting that discrete sites of close apposition between ER and mitochondria facilitate interorganelle calcium and phospholipid exchange.
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                Author and article information

                Journal
                Science
                Science
                American Association for the Advancement of Science (AAAS)
                0036-8075
                1095-9203
                February 08 2018
                February 09 2018
                February 09 2018
                January 18 2018
                : 359
                : 6376
                : 689-692
                10.1126/science.aar3607
                © 2018

                http://www.sciencemag.org/about/science-licenses-journal-article-reuse

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