Apontic binds the translational repressor Bruno and is implicated in regulation of oskar mRNA translation.
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The product of the oskar gene directs posterior patterning in the Drosophila oocyte, where it must be deployed specifically at the posterior pole. Proper expression relies on the coordinated localization and translational control of the oskar mRNA. Translational repression prior to localization of the transcript is mediated, in part, by the Bruno protein, which binds to discrete sites in the 3' untranslated region of the oskar mRNA. To begin to understand how Bruno acts in translational repression, we performed a yeast two-hybrid screen to identify Bruno-interacting proteins. One interactor, described here, is the product of the apontic gene. Coimmunoprecipitation experiments lend biochemical support to the idea that Bruno and Apontic proteins physically interact in Drosophila. Genetic experiments using mutants defective in apontic and bruno reveal a functional interaction between these genes. Given this interaction, Apontic is likely to act together with Bruno in translational repression of oskar mRNA. Interestingly, Apontic, like Bruno, is an RNA-binding protein and specifically binds certain regions of the oskar mRNA 3' untranslated region. (+info)
Vac1p coordinates Rab and phosphatidylinositol 3-kinase signaling in Vps45p-dependent vesicle docking/fusion at the endosome.
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The vacuolar protein sorting (VPS) pathway of Saccharomyces cerevisiae mediates transport of vacuolar protein precursors from the late Golgi to the lysosome-like vacuole. Sorting of some vacuolar proteins occurs via a prevacuolar endosomal compartment and mutations in a subset of VPS genes (the class D VPS genes) interfere with the Golgi-to-endosome transport step. Several of the encoded proteins, including Pep12p/Vps6p (an endosomal target (t) SNARE) and Vps45p (a Sec1p homologue), bind each other directly [1]. Another of these proteins, Vac1p/Pep7p/Vps19p, associates with Pep12p and binds phosphatidylinositol 3-phosphate (PI(3)P), the product of the Vps34 phosphatidylinositol 3-kinase (PI 3-kinase) [1] [2]. Here, we demonstrate that Vac1p genetically and physically interacts with the activated, GTP-bound form of Vps21p, a Rab GTPase that functions in Golgi-to-endosome transport, and with Vps45p. These results implicate Vac1p as an effector of Vps21p and as a novel Sec1p-family-binding protein. We suggest that Vac1p functions as a multivalent adaptor protein that ensures the high fidelity of vesicle docking and fusion by integrating both phosphoinositide (Vps34p) and GTPase (Vps21p) signals, which are essential for Pep12p- and Vps45p-dependent targeting of Golgi-derived vesicles to the prevacuolar endosome. (+info)
The exocyst is an effector for Sec4p, targeting secretory vesicles to sites of exocytosis.
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Polarized secretion requires proper targeting of secretory vesicles to specific sites on the plasma membrane. Here we report that the exocyst complex plays a key role in vesicle targeting. Sec15p, an exocyst component, can associate with secretory vesicles and interact specifically with the rab GTPase, Sec4p, in its GTP-bound form. A chain of protein-protein interactions leads from Sec4p and Sec15p on the vesicle, through various subunits of the exocyst, to Sec3p, which marks the sites of exocytosis on the plasma membrane. Sec4p may control the assembly of the exocyst. The exocyst may therefore function as a rab effector system for targeted secretion. (+info)
Cooperative binding of heat shock factor to the yeast HSP82 promoter in vivo and in vitro.
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Previous work has shown that heat shock factor (HSF) plays a central role in remodeling the chromatin structure of the yeast HSP82 promoter via constitutive interactions with its high-affinity binding site, heat shock element 1 (HSE1). The HSF-HSE1 interaction is also critical for stimulating both basal (noninduced) and induced transcription. By contrast, the function of the adjacent, inducibly occupied HSE2 and -3 is unknown. In this study, we examined the consequences of mutations in HSE1, HSE2, and HSE3 on HSF binding and transactivation. We provide evidence that in vivo, HSF binds to these three sites cooperatively. This cooperativity is seen both before and after heat shock, is required for full inducibility, and can be recapitulated in vitro on both linear and supercoiled templates. Quantitative in vitro footprinting reveals that occupancy of HSE2 and -3 by Saccharomyces cerevisiae HSF (ScHSF) is enhanced approximately 100-fold through cooperative interactions with the HSF-HSE1 complex. HSE1 point mutants, whose basal transcription is virtually abolished, are functionally compensated by cooperative interactions with HSE2 and -3 following heat shock, resulting in robust inducibility. Using a competition binding assay, we show that the affinity of recombinant HSF for the full-length HSP82 promoter is reduced nearly an order of magnitude by a single-point mutation within HSE1, paralleling the effect of these mutations on noninduced transcript levels. We propose that the remodeled chromatin phenotype previously shown for HSE1 point mutants (and lost in HSE1 deletion mutants) stems from the retention of productive, cooperative interactions between HSF and its target binding sites. (+info)
Correlation between protein and mRNA abundance in yeast.
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We have determined the relationship between mRNA and protein expression levels for selected genes expressed in the yeast Saccharomyces cerevisiae growing at mid-log phase. The proteins contained in total yeast cell lysate were separated by high-resolution two-dimensional (2D) gel electrophoresis. Over 150 protein spots were excised and identified by capillary liquid chromatography-tandem mass spectrometry (LC-MS/MS). Protein spots were quantified by metabolic labeling and scintillation counting. Corresponding mRNA levels were calculated from serial analysis of gene expression (SAGE) frequency tables (V. E. Velculescu, L. Zhang, W. Zhou, J. Vogelstein, M. A. Basrai, D. E. Bassett, Jr., P. Hieter, B. Vogelstein, and K. W. Kinzler, Cell 88:243-251, 1997). We found that the correlation between mRNA and protein levels was insufficient to predict protein expression levels from quantitative mRNA data. Indeed, for some genes, while the mRNA levels were of the same value the protein levels varied by more than 20-fold. Conversely, invariant steady-state levels of certain proteins were observed with respective mRNA transcript levels that varied by as much as 30-fold. Another interesting observation is that codon bias is not a predictor of either protein or mRNA levels. Our results clearly delineate the technical boundaries of current approaches for quantitative analysis of protein expression and reveal that simple deduction from mRNA transcript analysis is insufficient. (+info)
The Saccharomyces cerevisiae ETH1 gene, an inducible homolog of exonuclease III that provides resistance to DNA-damaging agents and limits spontaneous mutagenesis.
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The recently sequenced Saccharomyces cerevisiae genome was searched for a gene with homology to the gene encoding the major human AP endonuclease, a component of the highly conserved DNA base excision repair pathway. An open reading frame was found to encode a putative protein (34% identical to the Schizosaccharomyces pombe eth1(+) [open reading frame SPBC3D6.10] gene product) with a 347-residue segment homologous to the exonuclease III family of AP endonucleases. Synthesis of mRNA from ETH1 in wild-type cells was induced sixfold relative to that in untreated cells after exposure to the alkylating agent methyl methanesulfonate (MMS). To investigate the function of ETH1, deletions of the open reading frame were made in a wild-type strain and a strain deficient in the known yeast AP endonuclease encoded by APN1. eth1 strains were not more sensitive to killing by MMS, hydrogen peroxide, or phleomycin D1, whereas apn1 strains were approximately 3-fold more sensitive to MMS and approximately 10-fold more sensitive to hydrogen peroxide than was the wild type. Double-mutant strains (apn1 eth1) were approximately 15-fold more sensitive to MMS and approximately 2- to 3-fold more sensitive to hydrogen peroxide and phleomycin D1 than were apn1 strains. Elimination of ETH1 in apn1 strains also increased spontaneous mutation rates 9- or 31-fold compared to the wild type as determined by reversion to adenine or lysine prototrophy, respectively. Transformation of apn1 eth1 cells with an expression vector containing ETH1 reversed the hypersensitivity to MMS and limited the rate of spontaneous mutagenesis. Expression of ETH1 in a dut-1 xthA3 Escherichia coli strain demonstrated that the gene product functionally complements the missing AP endonuclease activity. Thus, in apn1 cells where the major AP endonuclease activity is missing, ETH1 offers an alternate capacity for repair of spontaneous or induced damage to DNA that is normally repaired by Apn1 protein. (+info)
The 3'-->5' exonucleases of DNA polymerases delta and epsilon and the 5'-->3' exonuclease Exo1 have major roles in postreplication mutation avoidance in Saccharomyces cerevisiae.
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Replication fidelity is controlled by DNA polymerase proofreading and postreplication mismatch repair. We have genetically characterized the roles of the 5'-->3' Exo1 and the 3'-->5' DNA polymerase exonucleases in mismatch repair in the yeast Saccharomyces cerevisiae by using various genetic backgrounds and highly sensitive mutation detection systems that are based on long and short homonucleotide runs. Genetic interactions were examined among DNA polymerase epsilon (pol2-4) and delta (pol3-01) mutants defective in 3'-->5' proofreading exonuclease, mutants defective in the 5'-->3' exonuclease Exo1, and mismatch repair mutants (msh2, msh3, or msh6). These three exonucleases play an important role in mutation avoidance. Surprisingly, the mutation rate in an exo1 pol3-01 mutant was comparable to that in an msh2 pol3-01 mutant, suggesting that they participate directly in postreplication mismatch repair as well as in other DNA metabolic processes. (+info)
A novel genetic screen for snRNP assembly factors in yeast identifies a conserved protein, Sad1p, also required for pre-mRNA splicing.
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The assembly pathway of spliceosomal snRNPs in yeast is poorly understood. We devised a screen to identify mutations blocking the assembly of newly synthesized U4 snRNA into a functional snRNP. Fifteen mutant strains failing either to accumulate the newly synthesized U4 snRNA or to assemble a U4/U6 particle were identified and categorized into 13 complementation groups. Thirteen previously identified splicing-defective prp mutants were also assayed for U4 snRNP assembly defects. Mutations in the U4/U6 snRNP components Prp3p, Prp4p, and Prp24p led to disassembly of the U4/U6 snRNP particle and degradation of the U6 snRNA, while prp17-1 and prp19-1 strains accumulated free U4 and U6 snRNA. A detailed analysis of a newly identified mutant, the sad1-1 mutant, is presented. In addition to having the snRNP assembly defect, the sad1-1 mutant is severely impaired in splicing at the restrictive temperature: the RP29 pre-mRNA strongly accumulates and splicing-dependent production of beta-galactosidase from reporter constructs is abolished, while extracts prepared from sad1-1 strains fail to splice pre-mRNA substrates in vitro. The sad1-1 mutant is the only splicing-defective mutant analyzed whose mutation preferentially affects assembly of newly synthesized U4 snRNA into the U4/U6 particle. SAD1 encodes a novel protein of 52 kDa which is essential for cell viability. Sad1p localizes to the nucleus and is not stably associated with any of the U snRNAs. Sad1p contains a putative zinc finger and is phylogenetically highly conserved, with homologues identified in human, Caenorhabditis elegans, Arabidospis, and Drosophila. (+info)