Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: UNIPROT:Q07644 (polypeptide)
 72,197 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

We have purified a nascent-polypeptide-associated complex (NAC) which prevents short ribosome-associated nascent polypeptides from inappropriate interactions with proteins in the cytosol. NAC binds nascent-polypeptide domains emerging from ribosomes unless a signal peptide is fully exposed. Depletion of cytosolic proteins (including NAC) from ribosomes carrying nascent polypeptides allows the signal recognition particle (SRP) to crosslink to polypeptides irrespective of whether or not they contain signal peptides. In the absence of cytosol, proteins lacking signal peptides can be mistranslocated into the endoplasmic reticulum in vitro, albeit with low efficiency. Readdition of NAC restores the specificity of SRP and fidelity of translocation.
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PMID:A protein complex required for signal-sequence-specific sorting and translocation. 804 62

Human Alu sequences are short interspersed DNA elements which have been greatly amplified by retrotransposition. Although initially derived from the 7SL RNA component of signal recognition particle (SRP), the Alu sequence has evolved into a dominant transposon while retaining a specific secondary structure found in 7SL RNA. We previously characterized a set of Alu sequences which are expressed as small cytoplasmic RNAs and isolated a protein that binds to these transcripts. Here we report that biochemical purification of this protein revealed it as the human homolog of the SRP 14 polypeptide which binds the Alu-homologous region of 7SL RNA. The human cDNA predicts an alanine-rich C-terminal tail translated from a trinucleotide repeat not found in the rodent homolog, which accounts for why the human protein-RNA complex migrates more slowly than its rodent counterpart in RNA mobility shift assays. The human Alu RNA-binding protein (RBP) is expressed after transfection of this cDNA into mouse cells. Expression of human RBP in rodent x human somatic cell hybrids is associated with substantial increase in endogenous small cytoplasmic Alu and scB1 transcripts but not other small RNAs. These studies provide evidence that this RBP associates with Alu transcripts in vivo and affects their metabolism and suggests a role for Alu transcripts in translation in an SRP-like manner. Analysis of hybrid lines indicated that the Alu RBP gene maps to human chromosome 15q22, which was confirmed by Southern blotting. The possibility that the primate-specific structure of this protein may have contributed to Alu evolution is considered.
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PMID:A human Alu RNA-binding protein whose expression is associated with accumulation of small cytoplasmic Alu RNA. 819 34

Translocation of proteins across the endoplasmic reticulum membrane is a GTP-dependent process. The signal recognition particle (SRP) and the SRP receptor both contain subunits with GTP binding domains. One GTP-dependent reaction during protein translocation is the SRP receptor-mediated dissociation of SRP from the signal sequence of a nascent polypeptide. Here, we have assayed the SRP and the SRP receptor for GTP binding and hydrolysis activities. GTP hydrolysis by SRP was not detected, so the maximal GTP hydrolysis rate for SRP was estimated to be < 0.002 mol GTP hydrolyzed x mol of SRP-1 x min-1. The intrinsic GTP hydrolysis activity of the SRP receptor ranged between 0.02 and 0.04 mol GTP hydrolyzed x mol of SRP receptor-1 x min-1. A 40-fold enhancement of GTP hydrolysis activity relative to that observed for the SRP receptor alone was obtained when complexes were formed between SRP and the SRP receptor. GTP hydrolysis activity was inhibited by GDP, but not by ATP. Extended incubation of the SRP or the SRP receptor with GTP resulted in substoichiometric quantities of protein-bound ribonucleotide. SRP-SRP receptor complexes engaged in GTP hydrolysis were found to contain a minimum of one bound guanine ribonucleotide per SRP-SRP receptor complex. We conclude that the GTP hydrolysis activity described here is indicative of one of the GTPase cycles that occur during protein translocation across the endoplasmic reticulum.
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PMID:GTP hydrolysis by complexes of the signal recognition particle and the signal recognition particle receptor. 822 41

The signal recognition particle (SRP) consists of one RNA and six protein subunits. The N-terminal domain of the 54K subunit contains a putative GTP-binding site, whereas the C-terminal domain binds signal sequences and SRP RNA. Binding of SRP to the signal sequence as it emerges from the ribosome creates a cytosolic targeting complex containing the nascent polypeptide chain, the translating ribosome, and SRP. This complex is directed to the endoplasmic reticulum membrane as a result of its interaction with the SRP receptor, a membrane protein composed of two subunits, SR alpha and SR beta, each of which also contains a GTP-binding domain. In the presence of GTP, SRP receptor binding to SRP causes the latter to dissociate from both the signal sequence and the ribosome. GTP is then hydrolysed so that SRP can be released from the SRP receptor and returned to the cytosol. Here we show that the 54K subunit (M(r) 54,000) of SRP (SRP54) is a GTP-binding protein stabilized in a nucleotide-free state by signal sequences, and that the SRP receptor both increases the affinity of SRP54 for GTP and activates its GTPase. We propose that nucleotide-mediated conformational changes in SRP54 regulate the release of signal sequences and the docking of ribosomes at the endoplasmic reticulum.
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PMID:GTP binding and hydrolysis by the signal recognition particle during initiation of protein translocation. 824 30

Ribosomes synthesizing nascent secretory proteins are targeted to the membrane by the signal recognition particle (SRP), a small ribonucleoprotein that binds to the signal peptide as it emerges from the ribosome. SRP arrests further elongation, causing ribosomes to stack behind the arrested ribosome. Upon interaction of SRP with its receptor on the ER membrane, the translation arrest is released and the ribosome becomes bound to the ER membrane. We have examined the distribution of unattached and membrane-bound ribosomes during the translation of mRNAs encoding two secretory proteins, bovine preprolactin and rat preproinsulin I. We find that the enhancement of ribosome stacking that occurs when SRP arrests translation of these proteins is relaxed in the presence of microsomal membranes. We also demonstrate that two previously described populations of membrane-associated ribosomes, distinguished by their sensitivity to high salt or EDTA extraction, correspond to ribosomes that have synthesized differing lengths of the nascent polypeptide. This analysis has revealed that nascent chain insertion into the membrane begins at distinct points for different presecretory proteins.
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PMID:Discrete nascent chain lengths are required for the insertion of presecretory proteins into microsomal membranes. 838 68

Proteins and RNAs homologous to components of the eukaryotic signal recognition particle (SRP) have been identified in a number of prokaryotic organisms. Here we report the isolation of an Arabidopsis thaliana cDNA, FFC (fifty-four chloroplast homologue), that encodes a chloroplast protein (54CP) homologous to the 54-kDa subunit of the signal recognition particle. 54CP shares 27% identity with mammalian SRP54 and 44% identity with the Escherichia coli ffh gene product suggesting a prokaryotic origin for this gene. FFC is a nuclear gene encoding a 62-kDa cytoplasmically synthesized precursor that is capable of being imported into isolated chloroplasts and processed to a 53-kDa stromal polypeptide. Antibodies generated against a portion of 54CP recognize an endogenous 53-kDa chloroplast protein that sediments at 4 S and 70 S, the latter form may be associated with ribosomes. The FFC transcript is most abundant in green shoot tissue whereas etiolated buds and roots have transcript levels about 30 and 10%, respectively, relative to light grown green shoots. By analogy with the eukaryotic signal recognition particle which targets secretory proteins to the endoplasmic reticulum, it is speculated that 54CP may target chloroplast proteins to either the thylakoid or envelope membranes.
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PMID:Characterization of a chloroplast homologue of the 54-kDa subunit of the signal recognition particle. 840 79

Many proteins, including the alpha subunit of the signal recognition particle receptor (SR alpha), are targeted within the cell by poorly defined mechanisms. A 140 residue N-terminal domain of SR alpha targets and anchors the polypeptide to the endoplasmic reticulum membrane by a mechanism independent of the pathway involving the signal recognition particle. To investigate the mechanism of membrane anchoring, translation pause sites on the SR alpha mRNA were used to examine the targeting of translation intermediates. A strong pause site at nucleotide 507 of the mRNA open reading frame corresponded with the shortest nascent SR alpha polypeptide able to assemble on membranes. An mRNA sequence at this pause site that resembles a class of viral -1 frameshift sequences caused translation pausing when transferred into another mRNA context. Site-directed mutagenesis of the mRNA greatly reduced translation pausing without altering the polypeptide sequence, demonstrating unambiguously a role for this mRNA sequence in translation pausing. SR alpha polypeptides synthesized from the non-pausing mRNA were impaired in co-translational membrane anchoring. Furthermore, co-translational membrane assembly of SR alpha appears to anchor polysomes translating SR alpha to membranes.
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PMID:The signal recognition particle receptor alpha subunit assembles co-translationally on the endoplasmic reticulum membrane during an mRNA-encoded translation pause in vitro. 859

Signal recognition particle-dependent targeting of secretory proteins to the endoplasmic reticulum membrane is predominant in the yeast Yarrowia lipolytica. A conditional lethal mutant of the SCR2-encoded 7S RNA provided the first in vivo evidence for involvement of this particle in cotranslational translocation (He, F., Beckerich, J. M., and Gaillardin, C. M. (1992) J. Biol. Chem. 267, 1932-1937). In order to identify partners of 7S RNA or signal recognition particle in their function, we selected synthetic lethal mutations with the 7S RNA mutation (sls). The SLS1 gene, cloned by complementation of the sls1 mutant growth defect, encodes a 426-amino acid polypeptide containing a NH2-terminal signal peptide and a COOH-terminal endoplasmic reticulum (ER) retention motif. The SLS1 gene product behaves as a lumenal protein of the ER. Sls1p was sedimented with membrane-rich organelles and was resistant to protease degradation without prior membrane solubilization. Immunofluorescence microscopy showed a typical endoplasmic reticulum perinuclear staining. Co-immunoprecipitation revealed that Sls1p resides close to the major translocation apparatus component, Sec61p. Deletion of the SLS1 gene led to a temperature-sensitive growth phenotype. Synthesis of several secretory proteins was shown to be specifically reduced in delta sls1 cells. We propose that Sls1p acts in the preprotein translocation process, interacting directly with translocating polypeptides to facilitate their transfer and/or help their folding in the ER.
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PMID:Sls1p, an endoplasmic reticulum component, is involved in the protein translocation process in the yeast Yarrowia lipolytica. 866 39

Proteins enter the secretory pathway by translocation across the endoplasmic reticulum (ER) membrane. In Saccharomyces cerevisiae, import of proteins into the ER occurs both cotranslationally and posttranslationally. Presumably, the cotranslational targeting to the ER membrane is directed by the signal recognition particle, as demonstrated in other eukaryotic systems. The deletion of a gene, called CER1, inhibits the translocation of proteins that enter the ER posttranslationally, but not those that enter cotranslationally. This translocation defect is more pronounced at lower temperatures. A strain possessing a null mutation of CER1 in combination with a kar2 temperature-sensitive mutation displays synthetic growth defects, whereas overexpression of the ER DnaJ homolog Scj1p suppresses the translocation defect in cer1Delta strains. CER1 is predicted to encode a 100-kDa polypeptide, residing in the ER lumen that is related to the hsp70 family of molecular chaperones.
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PMID:Cer1p, a novel Hsp70-related protein required for posttranslational endoplasmic reticulum translocation in yeast. 894 34

The interaction of protein SRP54M from the human signal recognition particle with SRP RNA was studied by systematic site-directed mutagenesis of the RNA molecule. Protein binding sites were identified by the analysis of mutations that removed individual SRP RNA helices or disrupted helical sections in the large SRP domain. The strongest effects on the binding activity of a purified polypeptide that corresponds to the methionine-rich domain of SRP54 (SRP54M) were caused by changes in helix 8 of the SRP RNA. Binding of protein SRP19 was diminished significantly by mutations in helix 6 and was stringently required for SRP54M to associate. Unexpectedly, mutant RNA molecules that resembled bacterial SRP RNAs were incapable of interaction with SRP54M, showing that protein SRP19 has an essential and direct role in the formation of the ternary complex with SRP54 and SRP RNA. Our findings provide an example for how, in eukaryotes, an RNA function has become protein dependent.
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PMID:Binding site of the M-domain of human protein SRP54 determined by systematic site-directed mutagenesis of signal recognition particle RNA. 901 69


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