In vivo N-glycosylation of the mep2 high-affinity ammonium transporter of Saccharomyces cerevisiae reveals an extracytosolic N-terminus

Saccharomyces cerevisiae possesses three related ammonium transporters, Mep1, Mep2 and Mep3, differing in their kinetic properties and in the level and regulation of their gene expression. The three Mep proteins belong to a family conserved in bacteria, plants and animals, which also includes proteins of the rhesus blood group family. In addition to its role in scavenging extracellular ammonium, the Mep2 protein has been proposed to act as an ammonium sensor, essential to pseudohyphal differentiation in response to ammonium limitation. To pursue the biochemical study of the Mep transporters, we raised polyclonal antibodies against the C-terminal tail of each Mep protein. When electrophoresed on SDS-polyacrylamide gel, the Mep1 and Mep3 proteins migrate as expected from their predicted size, whereas the Mep2 protein migrates as a high-molecular-weight smear. Protein deglycosylation with peptide-N-glycosidase F (PNGase F) indicates that, in contrast to Mep1 and Mep3, Mep2 is an asparagine-linked glycoprotein. Site-directed mutagenesis of the four potential N-glycosylation sites of Mep2 shows that Asn-4 of the protein's N-terminal tail is the only site that binds oligosaccharides. This provides evidence for the extracytosolic location of the Mep2 N-terminus. Consistently, treatment of intact protoplasts with proteinase K leads to specific proteolysis of the N-terminal tail of Mep2. The protein's C-terminus, on the other hand, is protected against protease degradation under these conditions, but digested after protoplast permeabilization, suggesting a cytoplasmic location for this part of the protein. Mep2 glycosylation is not required for pseudohyphal differentiation in response to ammonium starvation, and its absence causes only a slight reduction in the affinity of the transporter for its substrate.

A new family of high-affinity transporters for adenine, cytosine, and purine derivatives in Arabidopsis

In many organisms, including plants, nucleic acid bases and derivatives such as caffeine are transported across the plasma membrane. Cytokinins, important hormones structurally related to adenine, are produced mainly in root apices, from where they are translocated to shoots to control a multitude of physiological processes. Complementation of a yeast mutant deficient in adenine uptake (fcy2) with an Arabidopsis cDNA expression library enabled the identification of a gene, AtPUP1 (for Arabidopsis thaliana purine permease1), belonging to a large gene family (AtPUP1 to AtPUP15) encoding a new class of small, integral membrane proteins. AtPUP1 transports adenine and cytosine with high affinity. Uptake is energy dependent, occurs against a concentration gradient, and is sensitive to protonophores, potentially indicating secondary active transport. Competition studies show that purine derivatives (e.g., hypoxanthine), phytohormones (e.g., zeatin and kinetin), and alkaloids (e.g., caffeine) are potent inhibitors of adenine and cytosine uptake. Inhibition by cytokinins is competitive (competitive inhibition constant K(i) = 20 to 35 microM), indicating that cytokinins are transported by this system. AtPUP1 is expressed in all organs except roots, indicating that the gene encodes an uptake system for root-derived nucleic acid base derivatives in shoots or that it exports nucleic acid base analogs from shoots by way of the phloem. The other family members may have different affinities for nucleic acid bases, perhaps functioning as transporters for nucleosides, nucleotides, and their derivatives.

A new family of high-affinity transporters for adenine, cytosine, and purine derivatives in Arabidopsis

In many organisms, including plants, nucleic acid bases and derivatives such as caffeine are transported across the plasma membrane. Cytokinins, important hormones structurally related to adenine, are produced mainly in root apices, from where they are translocated to shoots to control a multitude of physiological processes. Complementation of a yeast mutant deficient in adenine uptake (fcy2) with an Arabidopsis cDNA expression library enabled the identification of a gene, AtPUP1 (for Arabidopsis thaliana purine permease1), belonging to a large gene family (AtPUP1 to AtPUP15) encoding a new class of small, integral membrane proteins. AtPUP1 transports adenine and cytosine with high affinity. Uptake is energy dependent, occurs against a concentration gradient, and is sensitive to protonophores, potentially indicating secondary active transport. Competition studies show that purine derivatives (e.g., hypoxanthine), phytohormones (e.g., zeatin and kinetin), and alkaloids (e.g., caffeine) are potent inhibitors of adenine and cytosine uptake. Inhibition by cytokinins is competitive (competitive inhibition constant K(i) = 20 to 35 microM), indicating that cytokinins are transported by this system. AtPUP1 is expressed in all organs except roots, indicating that the gene encodes an uptake system for root-derived nucleic acid base derivatives in shoots or that it exports nucleic acid base analogs from shoots by way of the phloem. The other family members may have different affinities for nucleic acid bases, perhaps functioning as transporters for nucleosides, nucleotides, and their derivatives.

A web site for the computational analysis of yeast regulatory sequences

A series of computer programs were developed for the analysis of regulatory sequences, with a special focus on yeast. These tools are publicly available on the web (http://copan.cifn.unam. mx/Computational_Biology/yeast-tools or http://www.ucmb.ulb.ac. be/bioinformatics/rsa-tools/). Basically, three classical problems can be addressed: (a) search for known regulatory patterns in the upstream regions of known genes; (b) discovery of unknown regulatory patterns within a set of upstream regions known to be co-regulated; (c) search for unknown genes potentially regulated by a known transcription factor. Each of these tasks can be performed on basis of a simple (string) or more refined (matrix) description of the regulatory patterns. A feature-map program automatically generates visual representations of the positions at which patterns were found. The site also provides a series of general utilities, such as generation of random sequence, automatic drawing of XY graphs, interconversions between sequence formats, etc. Several tools are linked together to allow their sequential utilization (piping), but each one can also be used independently by filling the web form with external data. This widens the scope of the site to the analysis of non-regulatory and/or non-yeast sequences.

Cross-talk between ammonium transporters in yeast and interference by the soybean SAT1 protein

Ammonium uptake in the yeast Saccharomyces cerevisiae involves three membrane transporters (Mep1, -2 and -3) belonging to an evolutionarily conserved protein family that also includes the rhesus (Rh) blood group polypeptides of erythrocytes. We show here that, in the 26972c mutant defective in NH4+ transport, the Mep1 protein carrying an amino acid substitution in its cytoplasmic C-terminus trans-inhibits the closely related Mep3 protein. The same mutation introduced into Mep3 leads to loss of transport activity and this inactive form also trans-inhibits native Mep3. Inhibition of Mep3 is post-translational and can be overcome by overexpression. These results are consistent with a direct interaction between Mep proteins, as is the case for the Rh polypeptides. The soybean GmSAT1 gene, recently cloned for its ability to complement the NH4+ transport defect of strain 26972c, has been described as an NH4+ channel protein involved in the transfer of fixed nitrogen from the bacteroid to the host plant. We show here that GmSAT1 contains a sequence homologous to the DNA-binding domain of basic helix-loop-helix (bHLH) transcription factors. We also show that GmSAT1 restores NH4+ uptake in the yeast mutant by interfering with the inhibition of Mep3. Our results are not consistent with a direct role of GmSAT1 in ammonium transport.

NH4+-induced down-regulation of the Saccharomyces cerevisiae Gap1p permease involves its ubiquitination with lysine-63-linked chains

Addition of ammonium ions to yeast cells growing on proline as the sole nitrogen source induces internalization of the general amino acid permease Gap1p and its subsequent degradation in the vacuole. An essential step in this down-regulation is Gap1p ubiquitination through a process requiring the Npi1p/Rsp5p ubiquitin ligase. We show in this report that NPI2, a second gene required for NH4+-induced down-regulation of Gap1p, codes for the ubiquitin hydrolase Doa4p/Ubp4p/Ssv7p and that NH4+-induced Gap1p ubiquitination is strongly reduced in npi2 cells. The npi2 mutation results in substitution of an aromatic amino acid located in a 33-residue sequence shared by some ubiquitin hydrolases of the Ubp family. In this mutant, as in doa4(delta) cells, the amount of free monomeric ubiquitin is at least four times lower than in wild-type cells. Both ubiquitination and down-regulation of the permease can be restored in npi2 cells by over-expression of ubiquitin. In proline-grown wild-type and npi2/doa4 cells overproducing ubiquitin, Gap1p appears to be mono-ubiquitinated at two lysine acceptor sites. Addition of NH4+ triggers rapid poly-ubiquitination of Gap1p, the poly-ubiquitin chains being specifically formed by linkage through the lysine 63 residue of ubiquitin. Gap1p is thus ubiquitinated differently from the proteins targeted by ubiquitination for proteolysis by the proteasome, but in the same manner as the uracil permease, also subject to ubiquitin-dependent endocytosis. When poly-ubiquitination through Lys63 is blocked, the Gap1p permease still undergoes NH4+-induced down-regulation, but to a lesser extent.

Transcriptional induction by aromatic amino acids in Saccharomyces cerevisiae

Aromatic aminotransferase II, product of the ARO9 gene, catalyzes the first step of tryptophan, phenylalanine, and tyrosine catabolism in Saccharomyces cerevisiae. ARO9 expression is under the dual control of specific induction and nitrogen source regulation. We have here identified UASaro, a 36-bp upstream element necessary and sufficient to promote transcriptional induction of reporter gene expression in response to tryptophan, phenylalanine, or tyrosine. We then isolated mutants in which UASaro-mediated ARO9 transcription is partially or totally impaired. Mutations abolishing ARO9 induction affect a gene called ARO80 (YDR421w), coding for a Zn2Cys6 family transcription factor. A sequence highly similar to UASaro was found upstream from the YDR380w gene encoding a homolog of bacterial indolepyruvate decarboxylase. In yeast, this enzyme is postulated to catalyze the second step of tryptophan catabolism to tryptophol. We show that ARO9 and YDR380w (named ARO10) have similar patterns of transcriptional regulation and are both under the positive control of Aro80p. Nitrogen regulation of ARO9 expression seems not directly to involve the general factor Ure2p, Gln3p, Nil1p, Uga43p, or Gzf3p. ARO9 expression appears, rather, to be mainly regulated by inducer exclusion. Finally, we show that Gap1p, the general amino acid permease, and Wap1p (Ycl025p), a newly discovered inducible amino acid permease with broad specificity, are the main aromatic amino acid transporters for catabolic purposes.

The yeast Npi1/Rsp5 ubiquitin ligase lacking its N-terminal C2 domain is competent for ubiquitination but not for subsequent endocytosis of the gap1 permease

The yeast ubiquitin ligase Npi1/Rsp5 and its mammalian homologue Nedd4 are involved in ubiquitination of various cell surface proteins, these being subsequently internalized by endocytosis and degraded in the vacuole/lysosome. Both enzymes consist of an N-terminal C2 domain, three to four successive WW(P) domains, and a C-terminal catalytic domain (HECT) containing a highly conserved cysteine residue involved in ubiquitin thioester formation. In this study, we show that the conserved cysteine of the HECT domain is required for yeast cell viability and for ubiquitination and subsequent endocytosis of the Gap1 permease. In contrast, the C2 domain of Npi1/Rsp5 is not essential to cell viability. Its deletion impairs internalization of Gap1, without detectably affecting ubiquitination of the permease. This suggests that Npi1/Rsp5 participates, via its C2 domain, in endocytosis of ubiquitinated permeases.

Amino acid signaling in Saccharomyces cerevisiae: a permease-like sensor of external amino acids and F-Box protein Grr1p are required for transcriptional induction of the AGP1 gene, which encodes a broad-specificity amino acid permease

The SSY1 gene of Saccharomyces cerevisiae encodes a member of a large family of amino acid permeases. Compared to the 17 other proteins of this family, however, Ssy1p displays unusual structural features reminiscent of those distinguishing the Snf3p and Rgt2p glucose sensors from the other proteins of the sugar transporter family. We show here that SSY1 is required for transcriptional induction, in response to multiple amino acids, of the AGP1 gene encoding a low-affinity, broad-specificity amino acid permease. Total noninduction of the AGP1 gene in the ssy1Delta mutant is not due to impaired incorporation of inducing amino acids. Conversely, AGP1 is strongly induced by tryptophan in a mutant strain largely deficient in tryptophan uptake, but it remains unexpressed in a mutant that accumulates high levels of tryptophan endogenously. Induction of AGP1 requires Uga35p(Dal81p/DurLp), a transcription factor of the Cys6-Zn2 family previously shown to participate in several nitrogen induction pathways. Induction of AGP1 by amino acids also requires Grr1p, the F-box protein of the SCFGrr1 ubiquitin-protein ligase complex also required for transduction of the glucose signal generated by the Snf3p and Rgt2p glucose sensors. Systematic analysis of amino acid permease genes showed that Ssy1p is involved in transcriptional induction of at least five genes in addition to AGP1. Our results show that the amino acid permease homologue Ssy1p is a sensor of external amino acids, coupling availability of amino acids to transcriptional events. The essential role of Grr1p in this amino acid signaling pathway lends further support to the hypothesis that this protein participates in integrating nutrient availability with the cell cycle.

A co-activator of nitrogen-regulated transcription in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, the transcription factors Gln3p and Nil1p of the GATA family play a determinant role in expression of genes that are subject to nitrogen catabolite repression. Here we report the isolation of a new yeast mutant, gan1-1, exhibiting dramatically decreased NAD-linked glutamate dehydrogenase (NAD-GDH) and glutamine synthetase (GS) activities. The GAN1 gene was cloned and found to encode a 488-amino-acid polypeptide bearing no typical DNA binding domain. Gan1p is required for full expression of GLN1, GDH2 and also other nitrogen utilization genes, including GAP1, PUT4, MEP2 and GDH1. The extent to which Gan1p is required, however, varies according to the gene and to the nitrogen source available. We show that Gan1p is in fact involved in Gln3p- and Nil1p-dependent transcription. In the case of Gln3p-dependent transcription, the degree to which Gan1p is required appears to be gene specific. The contribution of Gan1p to gene expression is also influenced by the nitrogen status of the cell. We found that GAN1 is identical to ADA1, which encodes a component of the ADA/GCN5 co-activator complex. Ada1/Gan1p thus represents the first reported case of an accessory protein (a co-activator) linking the GATA-binding proteins Gln3p and Nil1p, mediating nitrogen-regulated transcription, to the basal transcription machinery.

Extracting regulatory sites from the upstream region of yeast genes by computational analysis of oligonucleotide frequencies

We present here a simple and fast method allowing the isolation of DNA binding sites for transcription factors from families of coregulated genes, with results illustrated in Saccharomyces cerevisiae. Although conceptually simple, the algorithm proved efficient for extracting, from most of the yeast regulatory families analyzed, the upstream regulatory sequences which had been previously found by experimental analysis. Furthermore, putative new regulatory sites are predicted within upstream regions of several regulons. The method is based on the detection of over-represented oligonucleotides. A specificity of this approach is to define the statistical significance of a site based on tables of oligonucleotide frequencies observed in all non-coding sequences from the yeast genome. In contrast with heuristic methods, this oligonucleotide analysis is rigorous and exhaustive. Its range of detection is however limited to relatively simple patterns: short motifs with a highly conserved core. These features seem to be shared by a good number of regulatory sites in yeast. This, and similar methods, should be increasingly required to identify unknown regulatory elements within the numerous new coregulated families resulting from measurements of gene expression levels at the genomic scale. All tools described here are available on the web at the site http://copan.cifn.unam.mx/Computational_Biology/ yeast-tools

Nitrogen-regulated ubiquitination of the Gap1 permease of Saccharomyces cerevisiae

Addition of ammonium ions to yeast cells growing on proline as the sole nitrogen source induces rapid inactivation and degradation of the general amino acid permease Gap1 through a process requiring the Npi1/Rsp5 ubiquitin (Ub) ligase. In this study, we show that NH4+ induces endocytosis of Gap1, which is then delivered into the vacuole where it is degraded. This down-regulation is accompanied by increased conversion of Gap1 to ubiquitinated forms. Ubiquitination and subsequent degradation of Gap1 are impaired in the npi1 strain. In this mutant, the amount of Npi1/Rsp5 Ub ligase is reduced >10-fold compared with wild-type cells. The C-terminal tail of Gap1 contains sequences, including a di-leucine motif, which are required for NH4+-induced internalization and degradation of the permease. We show here that mutant Gap1 permeases affected in these sequences still bind Ub. Furthermore, we provide evidence that only a small fraction of Gap1 is modified by Ub after addition of NH4+ to mutants defective in endocytosis.

Benzene exposure in car mechanics and road tanker drivers

The purpose of this study was to identify professional factors related to benzene exposure and to deduce suitable safety measures. Atmospheric benzene, urinary muconic acid (tt-MA) and leukocyte alkaline phosphatase activity (LAPA) were evaluated among 66 car mechanics, 34 road tanker drivers, and 28 nonexposed workers. Professional and medical questionnaires were filled in at the same time. Atmospheric benzene was significantly higher among road tanker drivers than among car mechanics. The arithmetic mean +/- SD, median, and geometric mean values were, respectively, 0.48 +/- 1.49, 0.14, and 0.06 mg/m3 among car mechanics and 1.88 +/- 4.18, 0.68, and 0.65 mg/m3 among road tanker drivers. In the latter case the increase was caused by transport of unleaded petrol and correlated with the volume of the tank. Among car mechanics, tobacco smoking, windy conditions, dismantling of petrol filters, and handling of petrol increased atmospheric benzene levels. Urinary muconic acid was increased significantly among car mechanics (148 +/- 137, 127, and 111 micrograms/g) and among road tanker drivers (309 +/- 420, 137, and 151 micrograms/g) as compared with the controls (49 +/- 46, 33, and 33 micrograms/g). Among road tanker drivers, alcohol intake and transportation of unleaded petrol increased the excretion of muconic acid, which was also directly related to the volume of the tank. Among car mechanics, professional factors (dismantling of petrol filters, handling of and washing of hands with petrol) and nonprofessional factors (tobacco smoking and damaged skin on the hands and forearms) increased muconic acid excretion. In the control group, tobacco smoking increased its excretion. LAPA was not significantly modified among exposed workers. There was a weak but significant linear correlation between LAPA and muconic acid. These results suggest that to reduce exposure to benzene in unleaded petrol, individual and collective safety measures should be imposed in both occupations.

The nucleotide sequence of Saccharomyces cerevisiae chromosome XII

The yeast Saccharomyces cerevisiae is the pre-eminent organism for the study of basic functions of eukaryotic cells. All of the genes of this simple eukaryotic cell have recently been revealed by an international collaborative effort to determine the complete DNA sequence of its nuclear genome. Here we describe some of the features of chromosome XII.

The nucleotide sequence of Saccharomyces cerevisiae chromosome XIV and its evolutionary implications

In 1992 we started assembling an ordered library of cosmid clones from chromosome XIV of the yeast Saccharomyces cerevisiae. At that time, only 49 genes were known to be located on this chromosome and we estimated that 80% to 90% of its genes were yet to be discovered. In 1993, a team of 20 European laboratories began the systematic sequence analysis of chromosome XIV. The completed and intensively checked final sequence of 784,328 base pairs was released in April, 1996. Substantial parts had been published before or had previously been made available on request. The sequence contained 419 known or presumptive protein-coding genes, including two pseudogenes and three retrotransposons, 14 tRNA genes, and three small nuclear RNA genes. For 116 (30%) protein-coding sequences, one or more structural homologues were identified elsewhere in the yeast genome. Half of them belong to duplicated groups of 6-14 loosely linked genes, in most cases with conserved gene order and orientation (relaxed interchromosomal synteny). We have considered the possible evolutionary origins of this unexpected feature of yeast genome organization.

A C-terminal di-leucine motif and nearby sequences are required for NH4(+)-induced inactivation and degradation of the general amino acid permease, Gap1p, of Saccharomyces cerevisiae

The general amino acid permease, Gap1, of Saccharomyces cerevisiae is very active in cells grown on proline as the sole nitrogen source. Adding NH4+ to the medium triggers inactivation and degradation of the permease via a regulatory process involving Npi1p/Rsp5p, a ubiquitin-protein ligase. In this study, we describe several mutations affecting the C-terminal region of Gap1p that render the permease resistant to NH4(+)-induced inactivation. An in vivo isolated mutation (gap1pgr) causes a single Glu-->Lys substitution in an amino acid context similar to the DXKSS sequence involved in ubiquitination and endocytosis of the yeast alpha-factor receptor, Ste2p. Another replacement, substitution of two alanines for a di-leucine motif, likewise protects the Gap1 permease against NH4(+)-induced inactivation. In mammalian cells, such a motif is involved in the internalization of several cell-surface proteins. These data provide the first indication that a di-leucine motif influences the function of a plasma membrane protein in yeast. Mutagenesis of a putative phosphorylation site upstream from the di-leucine motif altered neither the activity nor the regulation of the permease. In contrast, deletion of the last eleven amino acids of Gap1p, a region conserved in other amino acid permeases, conferred resistance to NH4+ inactivation. Although the C-terminal region of Gap1p plays an important role in nitrogen control of activity, it was not sufficient to confer this regulation to two NH4(+)-insensitive permeases, namely the arginine (Can1p) and uracil (Fur4p) permeases.

Gzf3p, a fourth GATA factor involved in nitrogen-regulated transcription in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, two positive transcription factors of the GATA family, Gln3p and Nil1p/Gat1p, upregulate the expression of multiple nitrogen pathway genes via upstream 5'-GATA-3' sequences. Another GATA factor, Uga43p/Dal80p, downregulates to varying degrees the expression of some nitrogen-regulated genes. Here, we report the functional analysis of a fourth GATA factor, Gzf3p/Nil2p, whose gene was discovered by systematic sequencing of chromosome X. The Gzf3 protein most closely resembles Uga43p. Similar to Uga43p, Gzf3p has the properties of a negative GATA factor. While Uga43p is active specifically under nitrogen-depression conditions, Gzf3p exerts its negative regulatory function specifically on preferred nitrogen sources: It is involved in nitrogen repression of Nil1p-dependent transcription. At least one positive GATA factor is required for the UGA43 and GZF3 genes to be expressed. The Uga43p factor negatively regulates GZF3 expression and vice versa. In addition, both Uga43p and Gzf3p moderately regulate expression of their own genes. These two proteins seem to be parts of a complex network of GATA factors which probably play a determining role in nitrogen-regulated transcription.

Approach for the smoothing of three-dimensional reconstructions of the human spine using dual Kriging interpolation

In numerous situations, 3-D reconstructions of the spine are represented as curves in space, with the vertebral centroids as control points. Interpolation functions such as splines, polynomials or Fourier series have been used to minimise measurement errors and to perform specific calculations. A more general approach, dual Kriging, is presented which incorporates in a single formulation several methods, such as piece-wise linear interpolation, splines and least square functions as a limit case. To minimise user interaction and to control the different Kriging parameters, a computer program is developed allowing efficient use of these interpolation techniques in a clinical environment. Given different drift and covariance functions, the program determines the most suitable Kriging model for specific spine geometries and controls the amount of smoothing performed on raw data. Validation of the technique is with analytical 3-D curves, where random noise is added to represent reconstruction errors. A maximum absolute mean difference of 1.85 +/- 0.50 mm is found between the analytical and noisy curves smoothed with the Kriging technique for 200 points. Results obtained on actual 3-D reconstructions of scoliotic patients are very promising.

Dictyostelium discoideum contains a family of calmodulin-related EF-hand proteins that are developmentally regulated

A full-length genomic DNA fragment that codes for a novel EF-hand protein Dictyostelium discoideum was cloned and sequenced. The protein is composed of 168 amino acids and contains four consensus sequences that are typical for (Ca2+)-binding EF-hand domains. The protein sequence exhibits only minor similarities to other calmodulin-type proteins from Dictyostelium. The genomic DNA harbors two short introns; their positions suggest that the gene is unrelated to the EF-hand proteins from the calmodulin group. Northern blot analysis showed that the mRNA level was significantly increased during development. Polyclonal antibodies raised against the recombinant protein recognized in Western blots a protein of about 20 kDa. Like the mRNA, also the protein was more abundant in developing cells. Overlay experiments with 45Ca2+ indicated that the EF-hands in fact have (Ca2+)-binding activity. The recent description of CBP1, another calmodulin-type Dictyostelium protein that is upregulated during development [Coukell et al. (1995) FEBS Lett. 362, 342-346], suggests that D. discoideum contains a family of EF-hand proteins that have specific functions during distinct steps of development. We therefore designate the protein described in this report as CBP2.

The yeast YBR235w gene encodes a homolog of the mammalian electroneutral Na(+)-(K+)-C1- cotransporter family

The YBR235w gene of the yeast Saccharomyces cerevisiae was found during sequencing of chromosome II. Here, we show that the 1120 aa protein (Ybr235p) encoded by this gene shares strong sequence similarity with the highly related electroneutral Na(+)-C1- and Na(+)-K(+)-C1- cotransporters of animal cells. We hypothesize that this yeast protein also mediates active uptake of C1- into the cell.

NPl1, an essential yeast gene involved in induced degradation of Gap1 and Fur4 permeases, encodes the Rsp5 ubiquitin-protein ligase

When yeast cells growing on a poor nitrogen source are supplied with NH4+ ions, several nitrogen permeases including the general amino acid permease (Gap1p) are rapidly and completely inactivated. This report shows that inactivation by NH4+ of the Gap1 permease is accompanied by its degradation. A functional NPl1 gene product is required for both inactivation and degradation of Gap1p. Molecular analysis of the NPl1 gene showed that it is identical to RSP5. The RSP5 product is a ubiquitin-protein ligase (E3 enzyme) whose physiological function was, however, unknown. Its C-terminal region is very similar to that of other members of the E6-AP-like family of ubiquitin-protein ligases. Its N-terminal region contains a single C2 domain that may be a Ca(2+)-dependent phospholipid interaction motif, followed by several copies of a recently identified domain called WW(P). The Npi1/Rsp5 protein has a homologue both in humans and in mice, the latter being involved in brain development. Stress-induced degradation of the uracil permease (Fur4p), a process in which ubiquitin is probably involved, was also found to require a functional NPl1/RSP5 product. Chromosomal deletion of NPl1/RSP5 showed that this gene is essential for cell viability. In the viable npi1/rsp5 strain, expression of NPl1/RSP5 is reduced as a result of insertion of a Ty1 element in its 5' region. Our results show that the Npi1/Rsp5 ubiquitin-protein ligase participates in induced degradation of at least two permeases, Gap1p and Fur4p, and probably also other proteins.

Structure/function studies on cytoskeletal proteins in Dictyostelium amoebae as a paradigm

The actin cytoskeleton in motile non-muscle cells is being regulated by a large number of actin-binding proteins. A deeper insight into the complex nature of the dynamic rearrangements of the microfilament system during cell movement requires an experimental system that allows the combined application of biochemical, biophysical, cell biological and molecular methods. Dictyostelium amoebae are well suited especially for a genetic approach because they are amenable to gene disruption, antisense and gene tagging techniques. The actin-binding proteins profilin, hisactophilin and protovillin are being described in this context as typical examples that either bind to G-actin, or anchor the actin cytoskeleton to the plasma membrane, or are structurally similar to vertebrate proteins but distinct in their functions.

Radioimmunoassay for plasma histamine: a study of false positive and false negative values

In order to achieve a correct diagnosis of drug anaphylaxis using a radioimmunoassay devoid of interferences, we have studied factors leading to false positive or false negative values of plasma histamine. Different steps in sample collection were studied systematically in 30 normal volunteers. False positive values were found in haemolysed samples, with histamine concentrations being correlated with haemoglobin concentrations, and where plasma was aspirated from the white-cell layer. There was no significant increase when a tourniquet or vacuum tubes were used, or when blood tubes were left at 4 degrees C overnight. In 12 patients who experienced an anaphylactic reaction, histamine disappeared from blood 10 times more slowly than expected. False negative values were found in two pregnant women and one heparinized patient. Histamine was remarkably stable in vitro in blood or plasma samples, whereas it disappeared rapidly when plasma from a pregnant woman or a heparinized patient was added to the sample. We conclude that false positive and false negative values are rare when using this radioimmunoassay.