Cloning, pharmacology, and tissue distribution of G-protein-coupled receptor GPR105 (KIAA0001) rodent orthologs

It has recently been shown that UDP-glucose is a potent agonist of the orphan G-protein-coupled receptor (GPCR) KIAA0001. Here we report cloning and analysis of the rat and mouse orthologs of this receptor. In accordance with GPCR nomenclature, we have renamed the cDNA clone, KIAA0001, and its orthologs GPR105 to reflect their functionality as G-protein-coupled receptors. The rat and mouse orthologs show 80% and 83% amino acid identity, respectively, to the human GPR105 protein. We demonstrate by genomic Southern blot analysis that there are no genes in the mouse or rat genomes with higher sequence similarity. Chromosomal mapping shows that the mouse and human genes are located on syntenic regions of chromosome 3. Further analyses of the rat and mouse GPR105 proteins show that they are activated by the same agonists as the human receptor, responding to UDP-glucose and closely related molecules with similar affinities. The mouse and rat receptors are widely expressed, as is the human receptor. Thus we conclude that we have identified the rat and mouse orthologs of the human gene GPR105.

EDG1 receptor stimulation leads to cardiac hypertrophy in rat neonatal myocytes

Sphingosine 1 phosphate (S1P), an aminophospholipid, acts extracellularly as a ligand via the specific G protein-coupled receptors of the endothelial differentiation gene (EDG) 1, 3, 5, 6 and 8 receptors family and intracellularly as a second messenger in various cellular types. The aim of this work was to investigate biological activity of S1P in cardiomyocytes with respect to related sphingolipids. S1P was applied for 48 h on rat neonatal cardiomyocytes at 10 nM, 100 nM and 1 microM. S1P induced a concentration-dependent cellular hypertrophy evidenced by an increase in cell size, [3H]-phenylalanine incorporation, protein content and Brain Natriuretic Peptide (BNP) secretion. Among the lipids tested S1P exhibits the lower EC50 (67 nM) followed by dihydro-S1P (107 nM) and sphingosylphosphorylcholine (1.6 microM). The effect of S1P could be related to a stimulation of the EDG1 receptor since we showed that the EDG1 receptor is predominantly expressed at the mRNA and protein levels in rat cardiomyocytes and that specific anti-EDG1 antibodies inhibited the hypertrophic effect induced by S1P. Furthermore the expression level of most other EDG receptors for S1P appeared very low in cardiac myocytes. S1P (100 nM) increased the phosphorylation of p42/44MAPK, p38MAPK, JNK, Akt and p70(S6K), this effect being reversed by inhibitors of their respective phosphorylation which also rescue the hypertrophic phenotype. Finally, S1P stimulated actin stress fibre formation reverted by the Rho inhibitor, the C3 exoenzyme. Altogether, our results show that S1P induces cardiomyocyte hypertrophy mainly via the EDG1 receptor and subsequently via Gi through ERKs, p38 MAPK, JNK, PI3K and via Rho pathway.

Constitutive promoter modules for PCR-based gene modification in Saccharomyces cerevisiae

Initial steps in investigating gene function often include deleting and overexpressing the gene of interest and identifying the subcellular location of the gene product. To facilitate these procedures, a number of new PCR modules, which contain selectable markers and in some cases other genetic elements (e.g promoter elements, epitope tags, and reporter genes) have been developed. These modules are used as PCR substrates to create products that can be targeted to specified locations in the yeast genome, thus modifying that genomic locus. We describe here a series of plasmids that contain a truncated version of the strong ADH1 promoter with and without amino-terminal 3HA and GST tags. Because these plasmids contain the same vector sequences as the GAL1 promoter plasmids, a constitutive and an inducible promoter can now be integrated with a minimal number of primers.

Oligonucleotide-mediated, PCR-independent cloning by homologous recombination

We have developed an oligonucleotide-mediated cloning technique based on homologous recombination in Saccharomyces cerevisiae that allows precise DNA sequences to be transferred independent of restriction enzymes and PCR. In this procedure, linear DNA sequences are targeted to a chosen site in a yeast vector by DNA linkers, which consist of two partially overlapping oligonucleotides. The linkers contain relatively short regions of both yeast vector sequences and insert sequences, which stimulate homologous recombination between the vector and the insert. The linkers can also contain sequences not found in either the vector or the insert (e.g., sequences that encode ribosome binding sites, epitope tags, preferred codons, etc.), thus allowing modification of the transferred DNA. Linkers can be designed such that DNA sequences can be transferred with just two reusable universal oligonucleotides and two gene-specific oligonucleotides. This cloning method, which is performed by co-transforming yeast with linear vector, substrate DNA, and unannealed oligonucleotides, has been termed the yeast-based, oligonucleotide-mediated gap repair technique (YOGRT).

Characterization of human HtrA2, a novel serine protease involved in the mammalian cellular stress response

Human HtrA2 is a novel member of the HtrA serine protease family and shows extensive homology to the Escherichia coli HtrA genes that are essential for bacterial survival at high temperatures. HumHtrA2 is also homologous to human HtrA1, also known as L56/HtrA, which is differentially expressed in human osteoarthritic cartilage and after SV40 transformation of human fibroblasts. HumHtrA2 is upregulated in mammalian cells in response to stress induced by both heat shock and tunicamycin treatment. Biochemical characterization of humHtrA2 shows it to be predominantly a nuclear protease which undergoes autoproteolysis. This proteolysis is abolished when the predicted active site serine residue is altered to alanine by site-directed mutagenesis. In human cell lines, it is present as two polypeptides of 38 and 40 kDa. HumHtrA2 cleaves beta-casein with an inhibitor profile similar to that previously described for E. coli HtrA, in addition to an increase in beta-casein turnover when the assay temperature is raised from 37 to 45 degrees C. The biochemical and sequence similarities between humHtrA2 and its bacterial homologues, in conjunction with its nuclear location and upregulation in response to tunicamycin and heat shock suggest that it is involved in mammalian stress response pathways.

Temperature-sensitive differential affinity of TRAIL for its receptors. DR5 is the highest affinity receptor

TRAIL is a member of the tumor necrosis factor (TNF) family of cytokines which induces apoptotic cell death in a variety of tumor cell lines. It mediates its apoptotic effects through one of two receptors, DR4 and DR5, which are members of of the TNF receptor family, and whose cytoplasmic regions contain death domains. In addition, TRAIL also binds to 3 "decoy" receptors, DcR2, a receptor with a truncated death domain, DcR1, a glycosylphosphatidylinositol-anchored receptor, and OPG a secreted protein which is also known to bind to another member of the TNF family, RANKL. However, although apoptosis depends on the expression of one or both of the death domain containing receptors DR4 and/or DR5, resistance to TRAIL-induced apoptosis does not correlate with the expression of the "decoy" receptors. Previously, TRAIL has been described to bind to all its receptors with equivalent high affinities. In the present work, we show, by isothermal titration calorimetry and competitive enzyme-linked immunosorbent assay, that the rank order of affinities of TRAIL for the recombinant soluble forms of its receptors is strongly temperature dependent. Although DR4, DR5, DcR1, and OPG show similar affinities for TRAIL at 4 degrees C, their rank-ordered affinities are substantially different at 37 degrees C, with DR5 having the highest affinity (K(D) </= 2 nm) and OPG having the weakest (K(D) = 400 nm). Preferentially enhanced binding of TRAIL to DR5 was also observed at the cell surface. These results reveal that the rank ordering of affinities for protein-protein interactions in general can be a strong function of temperature, and indicate that sizeable, but hitherto unobserved, TRAIL affinity differences exist at physiological temperature, and should be taken into account in order to understand the complex physiological and/or pathological roles of TRAIL.

Saccharomyces cerevisiae Yak1p protein kinase autophosphorylates on tyrosine residues and phosphorylates myelin basic protein on a C-terminal serine residue

The serine/threonine protein kinase, Yak1p, functions as a negative regulator of the cell cycle in Saccharomyces cerevisiae, acting downstream of the cAMP-dependent protein kinase. In the present work we report that overexpression of haemagglutinin-tagged full-lengthYak1p and an N-terminally truncated form (residues 148-807) lead to growth arrest in PKA compromised yak1 null yeast cells. Both forms of recombinant Yak1p kinase were catalytically active and preferred myelin basic protein (MBP) as a substrate over several other proteins. Phosphopeptide analysis of bovine MBP by tandem MS revealed two major Yak1p phosphorylation sites, Thr-97 and Ser-164. Peptides containing each site were obtained and tested as Yak1p substrates. Both forms of Yak1p phosphorylated a peptide containing the Ser-164 residue with far more efficient kinetics than MBP. The maximal velocity (V(max)) values of the full-length Yak1p reaction were 110+/-21 (Ser-164) and 8.7+/-1.7 (MBP), and those of N-terminally truncated Yak1p were 560.7+/-74.8 (Ser-164) and 34. 4+/-2.2 (MBP) pmol/min per mg of protein. Although neither form of Yak1p was able to phosphorylate two generic protein tyrosine kinase substrates, both were phosphorylated on tyrosine residues in vivo and underwent tyrosine autophosphorylation when reacted with ATP in vitro. Tandem MS showed that Tyr-530 was phosphorylated both in vivo and in vitro after reaction with ATP. Pre-treatment with protein tyrosine phosphatase 1B removed all of Yak1p phosphotyrosine content and drastically reduced Yak1p activity against exogenous substrates, suggesting that the phosphotyrosine content of the enzyme is essential for its catalytic activity. Although the N-terminally truncated Yak1p was expressed at a lower level than the full-length protein, its catalytic activity and phosphotyrosine content were significantly higher than those of the full-length enzyme. Taken together, our results suggest that Yak1p is a dual specificity protein kinase which autophosphorylates on Tyr-530 and phosphorylates exogenous substrates on Ser/Thr residues.

A G protein-coupled receptor for UDP-glucose

Uridine 5'-diphosphoglucose (UDP-glucose) has a well established biochemical role as a glycosyl donor in the enzymatic biosynthesis of carbohydrates. It is less well known that UDP-glucose may possess pharmacological activity, suggesting that a receptor for this molecule may exist. Here, we show that UDP-glucose, and some closely related molecules, potently activate the orphan G protein-coupled receptor KIAA0001 heterologously expressed in yeast or mammalian cells. Nucleotides known to activate P2Y receptors were inactive, indicating the distinctly novel pharmacology of this receptor. The receptor is expressed in a wide variety of human tissues, including many regions of the brain. These data suggest that some sugar-nucleotides may serve important physiological roles as extracellular signaling molecules in addition to their familiar role in intermediary metabolism.

Acquisition of sensitivity of stress-activated protein kinases to the p38 inhibitor, SB 203580, by alteration of one or more amino acids within the ATP binding pocket

Pyridinyl imidazole inhibitors of p38 mitogen-activated protein kinase compete with ATP for binding. Mutation of 23 residues in the ATP pocket indicated that several residues which affected binding of pyridinyl imidazole photoaffinity cross-linker 125I-SB 206718 did not affect kinase activity, and vice versa, suggesting that pyridinyl imidazoles bind p38 differently than ATP. Two close homologues of p38, SAPK3 and SAPK4, are not inhibited by SB 203580 and differ from p38 by three amino acids near the hinge of the ATP pocket. Substitution of the three amino acids in p38 by those in SAPK3/4 (Thr-106, His-107, and Leu-108 to Met, Pro, and Phe) resulted in decreased 125I-SB 206718 cross-linking and loss of inhibition by SB 203580. Substitution of just Thr-106 by Met resulted in incomplete loss of inhibition. Conversely, substitution of the three amino acids of p38 into SAPK3, SAPK4, or the more distantly related JNK1 resulted in inhibition by SB 203580, whereas mutation of just Met-106 to Thr resulted in weaker inhibition. These results indicate that these three amino acids can confer specificity and sensitivity to SB 203580 for at least two different classes of MAPKs.

Role of conserved histidines in catalytic activity and inhibitor binding of human recombinant phosphodiesterase 4A

To identify critical amino acids within the central conserved region of recombinant human cAMP-specific phosphodiesterase 4 subtype A (rhPDE4A), we engineered the expression of point mutants in a fully active rhPDE4A/Met201-886. When histidine residues at positions 433, 437, 473, and 477, which are highly conserved among all PDE families, were changed independently to serine residues, cAMP hydrolyzing activities were substantially reduced or abolished. The ability of these mutants to bind prototypical PDE4 inhibitors [3H]-(R)-rolipram or [3H]RP 73401 was also decreased in parallel with the loss of catalytic activity. The parallel loss of catalytic activity and inhibitor binding suggests that these changes resulted from non-localized perturbations in the structure of the enzyme. More interesting results were obtained when histidine residues at positions 505 and 506 were changed independently to aspar agines. The K(m) value for cAMP increased 3-fold in H505N (K(m) = 11 +/- 3 microM) and 11-fold in H506N (K(m) = 44 +/- 6 microM) compared with the wild-type protein (K(m) = 4 +/- 1 microM). These mutant proteins bound [3H]-(R)-rolipram and [3H]RP 73401 with K(d) values of 1.8 +/- 0.4 and 0.3 +/- 0.1 nM, respectively, for H505N, and 3.9 +/- 0.9 and 0.5 +/- 0.1 nM, respectively, for H506N. These values are nearly identical to those obtained with the wild-type rhPDE4A/Met201-886. In contrast, the IC50 values for cAMP competition with either [3H]-(R)-rolipram or [3H]RP 73401 binding increased approximately 2-fold in H505N and approximately 13-fold in H506N compared with the wild type protein. These increases are virtually identical to the changes in the K(m) value for cAMP in these mutants. We conclude that His506 and, perhaps, His505 are involved in binding of cAMP to PDE4A/Met201-886 but not in binding of PDE4-selective inhibitors.

Pyridinyl imidazole inhibitors of p38 mitogen-activated protein kinase bind in the ATP site

The site of action of a series of pyridinyl imidazole compounds that are selective inhibitors of p38 mitogen-activated protein kinase in vitro and block proinflammatory cytokine production in vivo has been determined. Using Edman sequencing, 125I-SB206718 was shown to cross-link to the nonphosphorylated Escherichia coli-expressed p38 kinase at Thr175, which is proximal to the ATP binding site. Titration calorimetric studies with E. coli-expressed p38 kinase showed that SB203580 bound with a stoichiometry of 1:1 and that binding was blocked by preincubation of p38 kinase with the ATP analogue, FSBA (5'-[p-(fluorosulfonyl)benzoyl]adenosine), which covalently modifies the ATP binding site. The intrinsic ATPase activity of the nonphosphorylated enzyme was inhibited by SB203580 with a Km of 9.6 mM. Kinetic studies of active, phosphorylated yeast-expressed p38 kinase using a peptide substrate showed that SB203580 was competitive with ATP with a Ki of 21 nM and that kinase inhibition correlated with binding and biological activity. Mutagenesis indicated that binding of 125I-SB206718 was dependent on the catalytic residues K53 and D168 in the ATP pocket. These findings indicate that the pyridinyl imidazoles act in vivo by inhibiting p38 kinase activity through competition with ATP and that their selectivity is probably determined by differences in nonconserved regions within or near the ATP binding pocket.

Mapping the functional domains of human recombinant phosphodiesterase 4A: structural requirements for catalytic activity and rolipram binding

To identify functional domains of the 886-amino acid human recombinant cAMP-specific phosphodiesterase (PDE) subtype A (rhPDE4A), we engineered the expression of seven mutant proteins containing both NH2- and COOH-terminal truncations. The level of rhPDE4A protein expression in yeast was monitored by immunoblotting using enzyme-specific antisera. Biochemical profiles of the mutant proteins were compared with those of the full-length protein or a fully active truncated form of the enzyme (rhPDE4A Met265-886), lacking the first 264 amino acids. The smallest catalytically active fragment generated was Met332-722, which at 45 kDa is less than half the mass of the full-length enzyme (approximately 110 kDa) but spans the most highly conserved region of the PDE superfamily. Two prototypical PDE4 inhibitors, rolipram and RP 73401, inhibited cAMP hydrolyzing activity of all truncated forms of the enzyme, with IC50 values of 70-2000 nM and 0.2-0.6 nM, respectively. [3H](R)-Rolipram bound to two sites on Met265-886, a high affinity site (Kd1 = 0.7 +/- 0.3 nM) and a low affinity site (Kd2 = 34 +/- 10 nM). Interestingly, [3H](R)-rolipram failed to bind to Met332-886 with high affinity, indicating that high affinity binding is not required for inhibition of enzyme activity. Low affinity rolipram binding was still present in Met332-886 (Kd = 101 +/- 7 nM). In contrast to [3H](R)-rolipram, [3H]RP 73401 bound to a single class of high affinity sites on Met265-886 (Kd = 0.4 +/- 0.1 nM). Further truncation of the enzyme to Met332-886 had no effect on [3H]RP 73401 binding (Kd = 0.2 +/- 0.03 nM). We conclude that the catalytic center of rhPDE4A lies between amino acids 332 and 722. Furthermore, amino acids 265-332 may form a high affinity binding site for rolipram that is outside of the catalytic domain. As a more likely alternative, these amino acids may not form a distinct binding site but instead may be required for the recombinant enzyme to assume a conformation that binds rolipram at the catalytic domain with a high affinity.

Human platelet cGI-PDE: expression in yeast and localization of the catalytic domain by deletion mutagenesis

Cyclic adenosine monophosphate (cAMP) is an important modulator of platelet responses to agonists. Cyclic nucleotide phosphodiesterase (PDE) controls intracellular cAMP concentrations by hydrolyzing it to AMP. The major PDE activity in platelets is PDE3A (cyclic guanosine monophosphate [cGMP]-inhibited PDE). To obtain structural information on platelet PDE3A, we cloned the enzyme cDNA from a human erythroleukemia cell (HEL) library since the cell line expresses many platelet proteins. This clone consists of 87% of the full-length human myocardial PDE3A cDNA, spanning from nucleotides 456 to 4606, and is identical in sequence. The nucleotide coding for the N terminal 179 amino acid sequence (nt 1-536) as well as four other cDNAs (nt 1459-1632, nt 1765-1986, nt 2152-2538, and nt 2978-3375) obtained by RT-PCR of platelet RNA are also identical to the myocardial sequences, indicating that the HEL, myocardial, and platelet PDE3As are the same. Northern blot analysis of HEL cell RNA detected two mRNAs of 7.5 and 4.4 kb. Four new deletion mutants are reported. PDE 3A delta 1 and PDE 3A delta 2, encoding amino acids 665 to 1141 and amino acids 679 to 1141, respectively, were expressed in a PDE-deficient yeast. They displayed PDE activities of 172 and 79 pmol/mg/min, respectively. PDE 3A delta 3 and PDE 3A delta 4, encoding amino acids 686 to 1141 and 700 to 1141, had no detectable PDE activity. All mutant proteins were expressed as determined by Western blot analysis. These findings localize the PDE3A catalytic domain to within amino acid residues 679 to 1141.

Missense mutations at the FKBP12-rapamycin-binding site of TOR1

The TOR genes were first identified in Saccharomyces cerevisiae by the isolation of mutants which exhibit dominant resistance to the immunosuppressive and antifungal drug rapamycin (Rm). The originally characterized Rm-resistant (RmR) TOR1-1 and TOR2-1 alleles contain an Arg in place of a conserved Ser residue, which lies adjacent to the phosphatidylinositol (PI) kinase-related domain of TOR (Ser1972 in TOR1; Ser1975 in TOR2). Additional spontaneous RmR mutants containing Lys, Ile or Asn substitutions were subsequently isolated. As this Ser is a potential site for protein kinase C phosphorylation, we were interested in determining whether the observed RmR is due to steric hindrance of the FKBP12-Rm-TOR interaction or whether phosphorylation at this site is required to mediate the interaction. Using site-directed mutagenesis, we replaced the Ser1972 residue of TOR1 with either a conservative residue, Ala, an alternative potential phosphorylation site, Thr, or Asp to mimic phosphorylation. The TOR1 (S1972A) mutant protein retained Rm sensitivity (RmS), whereas both the Thr and Asp substitutions conferred RmR. RmS correlated with the ability to interact with FKBP12-Rm in a two-hybrid assay: both wild-type TOR1 and the S1972A mutant retained the ability to interact with FKBP12-Rm, whereas the S1972T, S1972D and S1972R mutants failed to interact. All mutant TOR1 proteins were able to complement the growth defect of tor1 null alleles, suggesting that the Ser1972 residue may not be required for TOR1 function in cycling cells. Since a TOR1(S1972A) mutant protein confers a RmS phenotype, interacts with FKBP12-Rm in a two-hybrid assay, and functions in vivo, we conclude that phosphorylation at Ser1972 is not necessary for the interaction between TOR1 and FKBP12-Rm.

Identification of mitogen-activated protein (MAP) kinase-activated protein kinase-3, a novel substrate of CSBP p38 MAP kinase

CSBP p38 is a mitogen-activated protein kinase that is activated in response to stress, endotoxin, interleukin 1, and tumor necrosis factor. Using a catalytically inactive mutant (D168A) of human CSBP2 as the bait in a yeast two-hybrid screen, we have identified and cloned a novel kinase which shares approximately 70% amino acid identity to mitogen-activated protein kinase-activated protein kinase (MAPKAP kinase)-2, and thus was designated MAPKAP kinase-3. The binding of CSBP to MAPKAP kinase-3 was confirmed in vitro by the precipitation of epitope-tagged CSBP1, CSBP2, and CSBP2(D168A) and endogenous CSBP from mammalian cells by a bacterially expressed GST-MAPKAP kinase-3 fusion protein and in vivo by co-precipitation of the epitope-tagged proteins co-expressed in HeLa cells. MAPKAP kinase-3 was phosphorylated by both CSBP1 and CSBP2 and was then able to phosphorylate HSP27 in vitro. Treatment of HeLa cells with sorbitol or TNF resulted in activation of CSBP and MAPKAP kinase-3 and activation of MAPKAP kinase-3 could be blocked by preincubation of cells with SB203580, a specific inhibitor of CSBP kinase activity. These data suggest that MAPKAP kinase-3 is activated by stress and cytokines and is a novel substrate of CSBP both in vitro and in vivo.

Aromatic amino-acid biosynthesis in Candida albicans: identification of the ARO4 gene encoding a second DAHP synthase

The primary step in the aromatic amino-acid biosynthetic pathway in Saccharomyces cerevisiae is catalyzed by two redundant isozymes of 3-deoxy-d-arabinoheptulosonate-7-phosphate (DAHP) synthase, either of which alone is sufficient to permit growth on synthetic complete media lacking aromatic acids (SC-Aro). The activity of one isozyme (encoded by the ARO3 gene) is feedback-inhibited by phenylalanine, whereas the activity of the other isozyme (encoded by the ARO4 gene) is feedback-inhibited by tyrosine. Transcription of both genes is controlled by GCN4. We previously cloned the ARO3 gene from the opportunistic pathogen Candida albicans and found that: (1) it can complement an aro3 aro4 double mutation in S. cerevisiae, an effect inhibited by excess phenylalanine; and (2) its expression is induced in response to amino-acid deprivation, consistent with the presence of two putative GCN4-responsive promoter elements (Pereira and Livi 1993, 1995). To determine whether other DAHP synthases exist in C. albicans, we have constructed a homozygous aro3-deletion mutant strain. Such a mutant was found to be phenotypically Aro+, i. e., capable of normal growth on SC-Aro media, suggesting the presence of at least one additional isozyme. To confirm this result, a 222-bp DNA fragment was amplified by the polymerase chain reaction (PCR) from genomic DNA prepared from the homozygous aro3-deletion mutant, using a degenerate primer based on a conserved N-terminal region of Aro3p plus a degenerate comeback primer encoding a conserved region of the protein that lies within the deleted portion of the gene. The nucleotide sequence of this PCR fragment predicts a 74-amino acid DAHP synthase-related protein which shows strong homology to Aro3p from S. cerevisiae and C. albicans, but even greater homology (78% identity) to S. cerevisiae Aro4p. We conclude that cells of C. albicans contain a second Aro4p-related DAHP synthase.

Prolonged beta adrenoceptor stimulation up-regulates cAMP phosphodiesterase activity in human monocytes by increasing mRNA and protein for phosphodiesterases 4A and 4B

Human peripheral blood monocytes were treated for 4 h with a combination of the beta-agonist salbutamol (3 microM) and the low-Km cAMP-specific phosphodiesterase (PDE4) inhibitor rolipram (30 microM) to produce a prolonged elevation of cAMP and consequent increase in PDE activity. After this treatment, isozyme-selective PDE inhibitors were used to characterize the cAMP PDE profiles of high-speed supernatants before and after DEAE-Sepharose column chromatography. These experiments, in which total soluble PDE activity was increased by 58%, showed that the increased PDE activity is due to up-regulation of PDE4 and that at least two of the four subtypes are up-regulated. Experiments in whole cells demonstrated that this relatively modest increase in PDE4 activity has significant functional consequences, reducing cAMP accumulation in response to both PGE2 and lower, though not maximal, concentrations of rolipram. Further characterization of PDE4 subtype expression in control and treated monocytes, using polymerase chain reaction and Western blotting with subtype-specific peptide antibodies, showed that resting monocytes express both mRNA and protein for PDE4A, PDE4B and PDE4D. The amount of message for PDE4A and PDE4B appeared to increase upon up-regulation, whereas mRNA for PDE4D was not detected in treated cells. Western blots showed increases in the amount of protein for both PDE4A and PDE4B after treatment. We conclude that the PDE4 subtypes are differentially regulated upon prolonged exposure to elevated cAMP, with the consequence that the PDE4 profiles of control and treated cells differ not only in total activity but also in the relative proportions of the subtypes represented.

Human mitogen-activated protein kinase CSBP1, but not CSBP2, complements a hog1 deletion in yeast

CSBP1 and CSBP2 are human homologues of the Saccharomyces cerevisiae Hog1 mitogen-activated protein kinase which is required for growth in high osmolarity media. Expression of CSBP1, but not CSBP2, complemented a hog1 delta phenotype. A CSBP2 mutant (A34V) that complements hog1 delta was isolated and found to have approximately 3-fold lower kinase activity than the wild-type CSBP2. Further analysis revealed that both the kinase activity and tyrosine phosphorylation of CSBP1 and CSBP2 (A34V) is regulated by salt. In contrast, wild-type CSBP2 is constitutively active but dependent on the upstream kinase, Pbs2. Mutagenesis studies showed that reduction or elimination of CSBP2 kinase activity restores salt responsiveness as measured by tyrosine phosphorylation suggesting that too high a level of kinase activity can result in desensitization of the host cell and inability to grow in high salt.

Characterization of two human cAMP-specific phosphodiesterase subtypes expressed in baculovirus-infected insect cells

Recombinant baculoviruses were constructed to express cDNAs encoding two distinct subtypes of human cAMP-specific phosphodiesterase (hPDE4A and hPDE4B). Infection of Spodoptera frugiperda insect cells with the appropriate recombinant baculoviruses resulted in high level production of biologically-active protein as measured by enzymatic activity and immunoblotting using subtype-specific anti-hPDE4 antisera. Both recombinant proteins showed catalytic activity with a low Km (approximately 3 microM) for cAMP (with no cGMP hydrolyzing activity) and were inhibited by R-rolipram with apparent Kis of 0.38 and 0.25 microM, respectively. The recombinant enzymes also contained saturable, stereoselective and high-affinity rolipram-binding sites (Kd approximately 2 nM). Thus, insect cell-derived hPDE4s possess kinetic properties analogous to native enzymes as well as to recombinant enzymes produced in yeast.

Candida albicans ALS1: domains related to a Saccharomyces cerevisiae sexual agglutinin separated by a repeating motif

Transfer of budding Candida albicans yeast cells from the rich, complex medium YEPD to the defined tissue culture medium RPMI 1640 (RPMI) at 37 degrees C and 5% CO2 causes rapid onset of hyphal induction. Among the genes induced under these conditions are hyphal-specific genes as well as genes expressed in response to changes in temperature, CO2 and specific media components. A cDNA library constructed from cells incubated for 20 min in RPMI was differentially screened with yeast (YEPD)- and hyphal (RPMI)-specific probes resulting in identification of a gene expressed in response to culture conditions but not regulated by the yeast-hyphal transition. The deduced gene product displays significant identity to Saccharomyces cerevisiae alpha-agglutinin, encoded by AG alpha 1, an adhesion glycoprotein that mediates mating of haploid cells. The presence of this gene in C. albicans is curious since the organism has not been observed to undergo meiosis. We designate the C. albicans gene ALS1 (for agglutinin-like sequence). While the N- and C-termini of the predicted 1260-amino-acid ALS1 protein resemble those of the 650-amino-acid AG alpha 1, ALS1 contains a central domain of tandem repeats consisting of a highly conserved 36-amino-acid sequence not present in AG alpha 1. These repeats are also present on the nucleotide level as a highly conserved 108 bp motif. Southern and Northern blot analyses indicate a family of C. albicans genes that contain the tandem repeat motif; at least one gene in addition to ALS1 is expressed under conditions similar to those for ALS1 expression. Genomic Southern blots from several C. albicans isolates indicate that the number of copies of the tandem repeat element in ALS1 differs across strains and, in some cases, between ALS1 alleles in the same strain, suggesting a strain-dependent variability in ALS1 protein size. Potential roles for the ALS1 protein are discussed.

A GCN-like response in Candida albicans

The control of amino acid and purine biosynthesis in the yeast Saccharomyces cerevisiae is mediated by the transcriptional activator GCN4. We previously identified the presence of two putative GCN4 responsive elements (GCREs) in the promoter sequence of the Candida albicans ARO3 gene, which encodes an enzyme in the aromatic amino acid pathway. We now show that amino acid deprivation results in a dramatic rise in the steady-state level of ARO3-specific mRNA, indicative of a GCN-like pathway in C. albicans.

The yeast FKS1 gene encodes a novel membrane protein, mutations in which confer FK506 and cyclosporin A hypersensitivity and calcineurin-dependent growth

FK506 and cyclosporin A (CsA) are potent immunosuppressive agents that display antifungal activity. They act by blocking a Ca(2+)-dependent signal transduction pathway leading to interleukin-2 transcription. Each drug forms a complex with its cognate cytosolic immunophilin receptor (i.e., FKBP12-FK506 and cyclophilin-CsA) which acts to inhibit the Ca2+/calmodulin-dependent protein phosphatase 2B, or calcineurin (CN). We and others have defined the Saccharomyces cerevisiae FKS1 gene by recessive mutations resulting in 100-1000-fold hypersensitivity to FK506 and CsA (as compared to wild type), but which do not affect sensitivity to a variety of other antifungal drugs. The fks1 mutant also exhibits a slow-growth phenotype that can be partially alleviated by exogenously added Ca2+ [Parent et al., J. Gen. Microbiol. 139 (1993) 2973-2984]. We have cloned FKS1 by complementation of the drug-hypersensitive phenotype. It contains a long open reading frame encoding a novel 1876-amino-acid (215 kDa) protein which shows no similarity to CN or to other protein phosphatases. The FKS1 protein is predicted to contain 10 to 12 transmembrane domains with a structure resembling integral membrane transporter proteins. Genomic disruption experiments indicate that FKS1 encodes a nonessential function; fks1::LEU2 cells exhibit the same growth and recessive drug-hypersensitive phenotypes observed in the original fks1 mutants. Furthermore, the fks1::LEU2 allele is synthetically lethal in combination with disruptions of both of the nonessential genes encoding the alternative forms of the catalytic A subunit of CN (CNA1 and CNA2). These data suggest that FKS1 provides a unique cellular function which, when absent, increases FK506 and CsA sensitivity by making the CNs (or a CN-dependent function) essential.

Interaction between FKBP12-rapamycin and TOR involves a conserved serine residue

The yeast TOR1 and TOR2 proteins were previously discovered as putative targets of the immunosuppressive drug rapamycin. Although their cellular function is unknown, they are predicted to be at least 215 kDa in size and possess a C-terminal phosphatidylinositol (PI) kinase-related domain. We previously identified a conserved Ser residue, within the PI kinase-related domain of both yeast TOR proteins (Ser1972 in TOR1; Ser1975 in TOR2), as being the site of missense mutations conferring dominant rapamycin resistance. The Ser1972/1975 residue of yeast TOR is conserved in mammalian TOR homologs. One possibility is that this residue is critical for a direct interaction between TOR and the FKBP12-rapamycin complex. There is very recent biochemical evidence for an interaction between mammalian TOR and FKBP12-rapamycin (Brown, E. J., Albers, M. W., Shin, T. B., Ichikawa, K., Keith, C. T., Lane, W. S., and Schreiber, S. L. (1994) Nature 369, 756-758; Sabatini, D. M., Erdjument-Bromage, H., Lui, M., Tempst, P., and Snyder, S. H. (1994) Cell 78, 35-43). Using the yeast two-hybrid system, we now have obtained genetic proof of a physical interaction between FKBP12-rapamycin and TOR and have demonstrated that this interaction requires the conserved Ser residue. We have found that a small fragment of wild-type yeast TOR2 spanning Ser1975 is capable of interacting with human FKBP12 in the presence of rapamycin, whereas an Arg1975 mutant fails to interact. This effect is dependent upon rapamycin and is antagonized by FK506.

A protein kinase involved in the regulation of inflammatory cytokine biosynthesis

Production of interleukin-1 and tumour necrosis factor from stimulated human monocytes is inhibited by a new series of pyridinyl-imidazole compounds. Using radiolabelled and radio-photoaffinity-labelled chemical probes, the target of these compounds was identified as a pair of closely related mitogen-activated protein kinase homologues, termed CSBPs. Binding of the pyridinyl-imidazole compounds inhibited CSBP kinase activity and could be directly correlated with their ability to inhibit cytokine production, suggesting that the CSBPs are critical for cytokine production.

A Candida albicans cyclic nucleotide phosphodiesterase: cloning and expression in Saccharomyces cerevisiae and biochemical characterization of the recombinant enzyme

We have cloned a Candida albicans gene, which encodes a cyclic nucleotide phosphodiesterase (PDEase), by complementation in a Saccharomyces cerevisiae PDEase-deficient mutant. The deduced amino acid sequence is similar to that of the low-affinity PDEase of S. cerevisiae (PDE1) and the cyclic nucleotide PDEase (PD) of Dictyostelium discoideum. Biochemical analysis of recombinant protein produced in S. cerevisiae indicated that the enzyme behaves as a PDE1 homologue: it hydrolyses both cAMP (Km = 0.49 mM) and cGMP (Km = 0.25 mM), does not require divalent cations for maximal activity and is only moderately inhibited by millimolar concentrations of standard PDEase inhibitors. Based on these data, we designate the C. albicans we have cloned, PDE1. Low-stringency genomic Southern blots showed cross-hybridization between C. albicans PDE1 and DNA from Candida stellatoidea, but not with DNA from S. cerevisiae or several closely related Candida species.

Yeast TOR (DRR) proteins: amino-acid sequence alignment and identification of structural motifs

The yeast TOR1 (DRR1) and TOR2 (DRR2) proteins are putative targets of the immunosuppressive drug rapamycin (Rm), defined by dominant drug-resistance mutations. They share a large C-terminal domain that exhibits sequence similarity to the 110-kDa subunit of phosphatidylinositol (PI) 3-kinases. In this report, we present an amino acid (aa) sequence alignment of TOR1 (DRR1) and TOR2 (DRR2) and identify conserved and nonconserved motifs within the N-terminal domain that are indicative of possible nuclear localization. We also show that the mutations responsible for Rm resistance in four independent drr2dom alleles alter the identical aa (Ser1975-->Arg) previously identified in drr1dom mutants (Ser1972-->Arg or Asn). Models for TOR (DRR) protein function are discussed.

Catalytic and ligand binding properties of the FK506 binding protein FKBP12: effects of the single amino acid substitution of Tyr82 to Leu

The binding of FK506 and rapamycin to their cytosolic receptor FKBP12 is an intermediate step in the paths leading to their potent immunosuppressive properties. One of the amino acids defining the hydrophobic binding cleft for the macrocycles is Tyr82, which is thought to form a hydrogen bond with the amide oxygens of the common pipecolyl structural element within the two macrolides. To understand better the influence of this amino acid residue in catalytic activity (cis-trans peptidyl prolyl isomerization) and ligand binding properties, a Tyr82 to Leu site-specific modification of FKBP12 was prepared, purified and characterized. Kinetic experiments have demonstrated that the Tyr82 to Leu modification has a greater effect on catalytic properties than on ligand binding affinities, a result which indicates that these inhibitors may not be binding as true transition-state analogues. In an additional test for cellular function, expression of both wild-type and mutant human FKBP12 in a strain of Saccharomyces cerevisiae rendered resistant to rapamycin by deletion of the gene encoding a cytosolic rapamycin binding protein (RPB1), the yeast homologue of FKBP12, restored wild-type drug sensitivity.

Cloning and expression of the ARO3 gene encoding DAHP synthase from Candida albicans

In Saccharomyces cerevisiae, the primary step in the aromatic (ARO) amino acid (aa) biosynthetic pathway is catalyzed by two isozymes of 3-deoxy-D-arabinoheptulosonate-7-phosphate synthase (DAHPS). The activity of one DAHPS isozyme (encoded by the ARO3 gene) is feedback inhibited by phenylalanine, whereas the other (encoded by the ARO4 gene) is inhibited by tyrosine. The expression of these genes is also regulated at the transcriptional level by the general control activator GCN4. We took advantage of the high degree of aa sequence homology between DAHPSs from several species to isolate ARO3 homologues from the pathogenic yeast Candida albicans. An ARO3/ARO4-specific sequence was generated from C. albicans genomic DNA by polymerase chain reaction amplification and used as a probe to screen a C. albicans cDNA library. A 1.3-kb cDNA clone was isolated and sequenced. The cDNA contains a long open reading frame predicting a 368-aa protein with significant homology to known DAHPSs, including both the S. cerevisiae ARO3 and ARO4 products (68.5% and 58.5% identity, respectively). Northern analysis of yeast and mycelial poly(A)+ RNA revealed equivalent expression of a 1.3-kb transcript in both cell types. A genomic clone was isolated by cross-hybridization, and analysis of the 5' untranslated region revealed the presence of a putative GCN4-binding site. This clone complemented an aro3 mutation in S. cerevisiae; functional complementation was inhibited by the presence of excess phenylalanine (but not tyrosine) in the growth medium, confirming that the cloned gene is the C. albicans homologue of ARO3.

Dominant missense mutations in a novel yeast protein related to mammalian phosphatidylinositol 3-kinase and VPS34 abrogate rapamycin cytotoxicity

Rapamycin is a macrolide antifungal agent that exhibits potent immunosuppressive properties. In Saccharomyces cerevisiae, rapamycin sensitivity is mediated by a specific cytoplasmic receptor which is a homolog of human FKBP12 (hFKBP12). Deletion of the gene for yeast FKBP12 (RBP1) results in recessive drug resistance, and expression of hFKBP12 restores rapamycin sensitivity. These data support the idea that FKBP12 and rapamycin form a toxic complex that corrupts the function of other cellular proteins. To identify such proteins, we isolated dominant rapamycin-resistant mutants both in wild-type haploid and diploid cells and in haploid rbp1::URA3 cells engineered to express hFKBP12. Genetic analysis indicated that the dominant mutations are nonallelic to mutations in RBP1 and define two genes, designated DRR1 and DRR2 (for dominant rapamycin resistance). Mutant copies of DRR1 and DRR2 were cloned from genomic YCp50 libraries by their ability to confer drug resistance in wild-type cells. DNA sequence analysis of a mutant drr1 allele revealed a long open reading frame predicting a novel 2470-amino-acid protein with several motifs suggesting an involvement in intracellular signal transduction, including a leucine zipper near the N terminus, two putative DNA-binding sequences, and a domain that exhibits significant sequence similarity to the 110-kDa catalytic subunit of both yeast (VPS34) and bovine phosphatidylinositol 3-kinases. Genomic disruption of DRR1 in a mutant haploid strain restored drug sensitivity and demonstrated that the gene encodes a nonessential function. DNA sequence comparison of seven independent drr1dom alleles identified single base pair substitutions in the same codon within the phosphatidylinositol 3-kinase domain, resulting in a change of Ser-1972 to Arg or Asn. We conclude either that DRR1 (alone or in combination with DRR2) acts as a target of FKBP12-rapamycin complexes or that a missense mutation in DRR1 allows it to compensate for the function of the normal drug target.

The tyrosine89 residue of yeast FKBP12 is required for rapamycin binding

Rapamycin (Rm) is a macrolide antifungal agent related to FK506 that exhibits potent immunosuppressive properties which are mediated through interaction with specific cytoplasmic receptors (FKBPs or RBPs, for FK506- and Rm-binding proteins, respectively). These proteins possess peptidyl-prolyl cis-trans isomerase (PPIase) activity in vitro which is inhibited by the binding of Rm and FK506. In Saccharomyces cerevisiae, Rm sensitivity (Rms) is mediated by binding of the drug to RBP1, a homolog of the 12-kDa human FK506-binding protein (FKBP12); null mutations in the yeast RBP1 gene result in a recessive drug resistance phenotype. To identify missense mutations that define amino acid (aa) residues in RBP1 involved in drug sensitivity, we selected and genetically characterized over 250 independent RmR rbp1 mutants and screened them for both RBP1-specific mRNA and protein expression. Whereas all rbp1 mutants expressed abundant levels of RBP1 mRNA, stable RBP1 protein production was detected in only one mutant strain. The RBP1 gene was PCR-generated (in triplicate) from several rbp1 mutants and independent clones were sequenced. Most of the immunoblot-negative alleles were found to contain various types of null mutations; however, some alleles contained specific missense mutations that apparently affect protein stability in vivo. The single immunoblot-positive allele was found to contain a mutation altering a specific residue (Tyr89) which is conserved among the known FKBPs, and which, based on the solution and x-ray structures of human FKBP12, has been proposed to be part of a hydrophobic drug-binding pocket for FK506 and Rm.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression of dopamine beta-hydroxylase in Drosophila Schneider 2 cells. Evidence for a mechanism of membrane binding other than uncleaved signal peptide

To characterize the mechanism of membrane attachment of dopamine beta-hydroxylase, an expression system producing the processed form of this enzyme has been developed. We have replaced the endogenous signal peptide of bovine dopamine beta-hydroxylase with a heterologous signal peptide which is efficiently recognized and cleaved in Drosophila Schneider 2 cells. A cDNA encoding this chimeric recombinant bovine enzyme has been stably transfected into Schneider 2 cells. The inducible expression of active dopamine beta-hydroxylase in these cells has been verified by Western blotting and enzyme activity assays. N-terminal sequence analysis of purified recombinant enzyme demonstrates complete removal of the signal peptide. Subcellular analysis shows that the recombinant enzyme exists as both a soluble and a membrane-bound form in these cells. These data demonstrate that the endogenous signal peptide is not required for the formation of the membranous dopamine beta-hydroxylase and further that the enzyme can be bound to membranes via a mechanism other than uncleaved signal sequence.

A low-Km, rolipram-sensitive, cAMP-specific phosphodiesterase from human brain. Cloning and expression of cDNA, biochemical characterization of recombinant protein, and tissue distribution of mRNA

We have isolated cDNA clones from human frontal cortex cDNA libraries that encode a unique subtype of the low-Km, cAMP-specific phosphodiesterases (PDEs IV). The 564-amino acid sequence of the protein (human brain PDE IV (hPDE IVB)) shows significant homology to a PDE IV subtype expressed in human monocytes (hPDE IVA), particularly within the approximately 300-amino acid PDE IV catalytic domain. The degree of protein sequence identity is much greater between hPDE IVB and a homolog derived from rat brain (92% over 562 amino acids) than between hPDE IVB and hPDE IVA (76% over 538 amino acids), suggesting a greater subtype-specific versus species-specific conservation of protein sequence. Analysis of the distribution of hPDE IVB mRNA expression revealed a restricted pattern, with an approximately 4-kilobase mRNA detected in brain, heart, lung, and skeletal muscle and not in placenta, liver, kidney, or pancreas. An additional approximately 5-kilobase hPDE IVB-related mRNA species was detected in brain tissue. Recombinant hPDE IVB displayed all of the expected kinetic characteristics for a PDE IV, including sensitivity to the isozyme-selective inhibitor rolipram (Ki = 0.085 microM). Scatchard analysis of (R)-[3H]rolipram binding data suggested the presence of two noninteracting high affinity rolipram-binding sites (Kd = 0.4 and 6 nM) or a negatively cooperative interaction among multiple binding sites.

Biochemical characteristics and cellular regulation of phosphodiesterase IV

Considerable interest has been generated in the potential utility of phosphodiesterase (PDE) IV inhibitors as a novel class of anti-asthmatic agents. Because a detailed understanding of the molecular and biochemical characteristics of any molecular target of interest provides a key ingredient for rational drug design, we cloned a cDNA encoding a PDE IV (hPDE IV) from a human monocyte library and expressed, purified and characterized the recombinant gene product. Purified hPDE IV has kinetic characteristics consistent with native PDE IV isolated from tissue sources. In addition, it is inhibited by rolipram (Ki = 60 nM) and other archetypical PDE IV-selective inhibitors. Purified hPDE IV also contains a high affinity binding site for rolipram (Kd = 2 nM), although the precise relationship between this site and the cAMP catalytic site is not clear. In other studies in which the regulation of PDE IV expression was examined in U937 cells, a human monocytic cell line, prolonged treatment with salbutamol was shown to induce an increase in the activity of PDE IV. This up-regulation of PDE IV activity appears to be mediated by cAMP and occurs at the transcriptional or pretranscriptional level. As a consequence of PDE IV up-regulation, the sensitivity of U937 cells to the inhibitory effects of adenylyl cyclase activators on cell function is greatly diminished. If such regulation of PDE IV occurs in inflammatory cells in vivo, it could have implications for the therapeutic use of beta-adrenoceptor agonists. Specifically, induction of PDE IV activity in asthmatics being treated with beta-adrenoceptor agonists could result in a heterologous desensitization of inflammatory cells to endogenous anti-inflammatory agents (e.g., epinephrine, prostaglandin E2).

Recombinant human secretory phospholipase A2: purification and characterization of the enzyme for active site studies

A secreted form of phospholipase A2 (PLA2) is thought to play an important role in inflammatory diseases. To characterize this enzyme the cDNA encoding a low molecular weight PLA2 was cloned from a human placental cDNA library. The cDNA encoding the human PLA2 was subcloned into an expression vector and subsequently transfected into Chinese hamster ovary (CHO) cells. A stable CHO cell clone, secreting ca 1 mg/L of recombinant PLA2 into the medium, was scaled up in culture to 180 L. The recombinant enzyme was purified from the cell supernatant to apparent homogeneity by a novel procedure combining adsorption to poly(vinylidene difluoride) membranes, ion exchange chromatography and size exclusion chromatography. The final recovery of PLA2 activity was 58%. A direct comparison between the purified recombinant human PLA2 and PLA2 purified from human synovial fluid, including molecular weight, antigenicity, ionic dependence, substrate specificity and sensitivity to known PLA2 inhibitors, indicated that the two enzymes exhibit identical biochemical properties. These results show that the recombinant PLA2 can be efficiently expressed and purified in sufficient quantities to characterize the enzyme active site, to aid in the rational development of PLA2 inhibitors as potential anti-inflammatory drugs, and to investigate further the role of PLA2 in inflammatory disease.

Cloning and sequence analysis of a rapamycin-binding protein-encoding gene (RBP1) from Candida albicans

Rapamycin (Rm) and FK506 are macrolide antifungal agents that exhibit potent immunosuppressive properties in higher eukaryotes which are mediated through interaction with specific receptor proteins (FKBPs or RBPs, for FK506- and Rm-binding proteins, respectively). These proteins possess peptidyl-prolyl cis-trans isomerase (PPIase) activity in vitro which is inhibited by the binding of Rm and FK506. We previously isolated a gene encoding an RBP from Saccharomyces cerevisiae, and demonstrated that null mutations in this gene (called RBP1) result in a recessive Rm-resistant (RmR) phenotype. We now have cloned the Candida albicans RBP1 gene via complementation of the RmR phenotype in S. cerevisiae. The predicted C. albicans RBP exhibits 61%, 52% and 49% amino acid (aa) sequence identity with RBPs (FKBPs) from S. cerevisiae, Neurospora crassa and human cells (FKBP-12), respectively. Furthermore, several of the aa residues identified as being important for drug binding in human FKBP-12 are conserved within the C. albicans RBP.

The yeast cyclophilin multigene family: purification, cloning and characterization of a new isoform

Cyclophilins (Cyps) constitute a highly conserved family of proteins present in a wide variety of organisms. Historically, Cyps were first identified by their ability to bind the immunosuppressive agent cyclosporin A (CsA) with high affinity; they later were found to have peptidyl-prolyl cis-trans isomerase (PPIase) activity, which catalyzes the folding of oligopeptides at proline-peptide bonds in vitro and may be important for protein folding in vivo. Cells of Saccharomyces cerevisiae contain at least two distinct Cyp-related PPIases encoded by the genes CYP1 and CYP2. A yeast strain (GL81) containing genomic disruptions of three known yeast PPIase-encoding genes [CYP1, CYP2 and RBP1 (for rapamycin-binding protein); Koltin et al., Mol. Cell. Biol. 11 (1991) 1718-1723] was previously constructed and found to be viable. Soluble fractions of these cells possess residual CsA-sensitive PPIase activity (2-5% of that present in wild-type cells as assayed in vitro). We have purified an approx. 18-kDa protein exhibiting PPIase activity from a soluble fraction of GL81 cells and determined that its N-terminal amino acid (aa) sequence exhibits significant homology (but nonidentity) to the Cyp1 and Cyp2 proteins. We designate the gene for this new protein, CYP3. Using a degenerate oligodeoxyribonucleotide (oligo) based on the N-terminal aa sequence, plus an internal oligo homologous to a conserved region within the portion of CYP1 and CYP2 that had been deleted in the genome, a CYP3-specific DNA fragment was generated by the polymerase chain reaction (PCR) using GL81 genomic DNA as a substrate. This PCR fragment was used as a probe to isolate CYP3 genomic and cDNA clones.(ABSTRACT TRUNCATED AT 250 WORDS)

Coexpression of human cAMP-specific phosphodiesterase activity and high affinity rolipram binding in yeast

Studies by various investigators have demonstrated that the low Km, cAMP-specific phosphodiesterase (PDE IV) is selectively inhibited by a group of compounds typified by rolipram and Ro 20-1724. In addition to inhibiting the catalytic activity of PDE IV, rolipram binds to a high affinity binding site present in brain homogenates. Although it has been assumed that the high affinity rolipram-binding site is PDE IV, no direct evidence has been produced to support this assumption. The present studies were undertaken to determine whether the rolipram-binding site is coexpressed with PDE IV catalytic activity in Saccharomyces cerevisiae genetically engineered to express human recombinant monocytic PDE IV (hPDE IV). Expressing hPDE IV cDNA in yeast resulted in a 20-fold increase in PDE activity that was evident within 1 h of induction and reached a maximum by 3-6 h. The recombinant protein represented hPDE IV as judged by its immunoreactivity, molecular mass (approximately 88 kDa), kinetic characteristics (cAMP Km = 3.1 microM; cGMP Km greater than 100 microM), sensitivity to rolipram (Ki = 0.06 microM), and insensitivity to siguazodan (PDE III inhibitor) and zaprinast (PDE V inhibitor). Saturable, high affinity [3H] (R)-rolipram-binding sites (Kd = 1.0 nM) were coexpressed with PDE activity, indicating that both binding activity and catalytic activity are properties of the same protein. A limited number of compounds were tested for their ability to inhibit hPDE IV catalytic activity and compete for [3H](R)-rolipram binding. Analysis of the data revealed little correlation (r2 = 0.35) in the structure-activity relationships for hPDE IV inhibition versus competition for [3H] (R)-rolipram binding. In fact, certain compounds (e.g. (R)-rolipram Ro 20-1724) possessed a 10-100-fold selectivity for inhibition of [3H] (R)-rolipram binding over hPDE IV inhibition, whereas others (e.g. dipyridamole, trequinsin) possessed a 10-fold selectivity for PDE inhibition. Thus, although the results of these studies demonstrate that hPDE IV activity and high affinity [3H](R)-rolipram binding are properties of the same protein, they do not provide clear cut evidence linking the binding site with the PDE inhibitory activity of rolipram and related compounds.

The cyclophilin multigene family of peptidyl-prolyl isomerases. Characterization of three separate human isoforms

Cyclophilin (CyP), a major cytosolic protein possessing peptidyl-prolyl cis-trans isomerase activity, has been implicated as the specific receptor of the immunosuppressive drug cyclosporin A (CsA). To identify other potential CsA receptors related to CyP, two human cDNA libraries were screened under low stringency conditions using human CyP cDNA (encoding hCyP1) as a probe. Two cDNAs were identified which encode distinct proteins related to human hCyP1. These two novel proteins, designated hCyP2 and hCyP3, share 65 and 76% amino acid sequence homology with hCyP1, respectively. Both hCyP2 and hCyP3 contain NH2-terminal hydrophobic extensions of 32 and 42 amino acids, respectively. Protein-specific antibodies revealed the predominant association of hCyP2 and hCyP3 with membranes and subcellular organelles, which suggests that the amino-terminal leader sequences of the two CyP isoforms may act as signal peptides. In contrast to the results with hCyP1, Southern blot analysis indicated that both hCyP2 and hCyP3 gene sequences are represented infrequently in the human genome. Northern and Western blot analysis showed that the distribution of mRNA and proteins of the three hCyPs in differing tissues and cell types was similar. Each hCyP protein was expressed in Escherichia coli, purified, and shown to be an active peptidyl-prolyl isomerase. Substrate specificity was examined with 11 synthetic peptides (Suc-Xaa-Yaa-Pro-Phe-4-nitroanilide), and inhibition of the peptidyl-prolyl isomerase activities associated with hCyP1, hCyP2, and hCyP3 was studied with CsA, MeAla6-CsA and MeBm2t1-CsA. From both equilibrium considerations and the results of kinetic characterizations it is proposed that of these three CyP proteins, hCyP1 is the most likely intracellular target for CsA.

The CYP2 gene of Saccharomyces cerevisiae encodes a cyclosporin A-sensitive peptidyl-prolyl cis-trans isomerase with an N-terminal signal sequence

Cells of Saccharomyces cerevisiae contain a major cytosolic cyclophilin (Cyp)-related peptidyl-prolyl cis-trans isomerase (PPIase) which is the target for cyclosporin A (CsA) cytotoxicity and which is encoded by the CYP1 gene [Haendler et al., Gene 83 (1989) 39-46]. We recently identified a second Cyp-related gene in yeast, CYP2 [Koser et al., Nucleic Acids Res. 18 (1990) 1643] which predicts a protein with a hydrophobic leader sequence. A sequence lacking 33 codons from the 5'-end of the CYP2 open reading frame was generated by the polymerase chain reaction and engineered for expression in Escherichia coli. The corresponding recombinant truncated protein was purified and found to exhibit PPIase activity which was inhibited by CsA. The CYP2 gene is genetically unlinked to CYP1. As with CYP1, genomic disruption of CYP2 had no effect on haploid cell viability. Disruption of all three of the known yeast PPIase-encoding genes [CYP1, CYP2, and RBP1 for rapamycin-binding protein; Koltin et al., Mol. Cell. Biol. 11 (1991) 1718-1723] in the same haploid cell also resulted in no apparent cellular phenotype, suggesting either that none of these enzymes have an essential function or that additional PPIases can compensate for their specific absence. Whereas cells containing a genomic disruption of CYP1 exhibited a CsA-resistant phenotype, genomic disruption of CYP2 had no effect on CsA sensitivity. This suggests that the CYP1 gene product is the primary cellular target for CsA toxicity in yeast. Since both purified Cyps display CsA sensitivity in vitro, our data suggest that Cyp1 and Cyp2 differ in terms of their cellular function and/or localization.

Use of polymerase chain reaction for rapid detection of gene insertions in whole yeast cells

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Secretion of N-glycosylated interleukin-1 beta in Saccharomyces cerevisiae using a leader peptide from Candida albicans. Effect of N-linked glycosylation on biological activity

Human interleukin-1 beta (IL-1 beta) is expressed in activated monocytes as a 31-kDa precursor protein which is processed and secreted as a mature, unglycosylated 17-kDa carboxyl-terminal fragment, despite the fact that it contains a potential N-linked glycosylation site near the NH2 terminus (-Asn7-Cys8-Thr9-). cDNA coding for authentic mature IL-1 beta was fused to the signal sequence from the Candida albicans glucoamylase gene, two amino acids downstream from the signal processing site. Upon expression in Saccharomyces cerevisiae, approximately equimolar amounts of N-glycosylated (22 kDa) and unglycosylated (17 kDa) IL-1 beta protein were secreted. The N-glycosylated yeast recombinant IL-1 beta exhibited a 5-7-fold lower specific activity compared to the unglycosylated species. The mechanism responsible for inefficient glycosylation was also studied. We found no differences in secretion kinetics or processing between the two extracellular forms of IL-1 beta. The 17-kDa protein, which was found to lack core sugars, does not result from deglycosylation of the 22-kDa protein in vivo and does not result from saturation of the glycosylation enzymatic machinery through overexpression. Alteration of the uncommon Cys8 residue in the -Asn-X-Ser/Thr-glycosylation site to Ser also had no effect. However, increasing the distance between Asn7 and the signal processing site increased the extent of core N-linked glycosylation, suggesting a reduction in glycosylation efficiency near the NH2 terminus.

Expression of human recombinant cAMP phosphodiesterase isozyme IV reverses growth arrest phenotypes in phosphodiesterase-deficient yeast

The low-Km cAMP-specific phosphodiesterases (PDEases) are of great pharmacological significance because of their involvement in regulating cAMP concentrations, which, in turn, are responsible for mediating the cellular response to extracellular signals such as hormones and neurotransmitters. We recently reported the isolation of a cDNA clone that encodes a human monocyte low-Km, rolipram-sensitive, cAMP PDEase (isozyme IV). We have engineered the inducible expression of this human PDEase in yeast. Cells of Saccharomyces cerevisiae contain two genes that encode cAMP PDEases. PDEase-deficient mutants are viable but exhibit specific growth arrest phenotypes associated with elevated intracellular cAMP content; these phenotypes include heat shock sensitivity and the inability to grow on acetate as a carbon source. We show that functional expression of our human cAMP PDEase in a genetically engineered PDEase-deficient strain of S. cerevisiae reverses these aberrant phenotypes. Furthermore, under conditions for growth arrest, rolipram is cytotoxic to PDEase-deficient mutants expressing the human cAMP PDEase, indicating that it is capable of inhibiting the human recombinant enzyme in vivo. This system can be used in the development of a yeast cell-based assay for isozyme-selective inhibitors of the human recombinant cAMP PDEase.

A Candida albicans homolog of a human cyclophilin gene encodes a peptidyl-prolyl cis-trans isomerase

A Candida albicans cDNA and its genomic counterpart were isolated from lambda phage libraries using a human T-cell cyclophilin (Cyp) cDNA as a hybridization probe. The clones contain a 486-bp open reading frame predicting a 162-amino acid, approx. 18 kDa protein which is similar in size to, and which shares 68 and 81% homology with, human T-cell Cyp and cytosolic Saccharomyces cerevisiae Cyp, respectively. Northern blots show the presence of a single mRNA species of about 800 bp. However, genomic Southern blots suggest the presence of at least one other Cyp-related gene in C. albicans. The cDNA was engineered for expression in Escherichia coli, and the resulting recombinant protein, like mammalian Cyps, exhibited a peptidyl-prolyl cis-trans isomerase (PPIase) activity which was sensitive to inhibition by cyclosporin A in vitro. These results indicate that the gene which we have cloned encodes a C. albicans Cyp. We designate this gene CYP1 (cyclophilin). Interestingly, the predicted C. albicans protein contains only two cysteine residues which do not align with any of the four cysteines conserved among mammalian Cyps. This suggests that the PPIase catalytic mechanism may not involve an enzyme-bound hemithioorthoamide, as previously reported for porcine Cyp.

Cloning and expression of cDNA for a human low-Km, rolipram-sensitive cyclic AMP phosphodiesterase

We have isolated cDNA clones representing cyclic AMP (cAMP)-specific phosphodiesterases (PDEases) from a human monocyte cDNA library. One cDNA clone (hPDE-1) defines a large open reading frame of ca. 2.1 kilobases, predicting a 686-amino-acid, ca. 77-kilodalton protein which contains significant homology to both rat brain and Drosophila cAMP PDEases, especially within an internal conserved domain of ca. 270 residues. Amino acid sequence divergence exists at the NH2 terminus and also within a 40- to 100-residue domain near the COOH-terminal end. hPDE-1 hybridizes to a major 4.8-kilobase mRNA transcript from both human monocytes and placenta. The coding region of hPDE-1 was engineered for expression in COS-1 cells, resulting in the overproduction of cAMP PDEase activity. The hPDE-1 recombinant gene product was identified as a low-Km cAMP phosphodiesterase on the basis of several biochemical properties including selective inhibition by the antidepressant drug rolipram. Known inhibitors of other PDEases (cGMP-specific PDEase, cGMP-inhibited PDEase) had little or no effect on the hPDE-1 recombinant gene product. Human genomic Southern blot analysis suggests that this enzyme is likely to be encoded by a single gene. The presence of the enzyme in monocytes may be important for cell function in inflammation. Rolipram sensitivity, coupled with homology to the Drosophila cAMP PDEase, which is required for learning and memory in flies, suggests an additional function for this enzyme in neurobiochemistry.

Extragenic suppressors of mar2(sir3) mutations in Saccharomyces cerevisiae

The silent mating-type genes (HML and HMR) of Saccharomyces cerevisiae are kept under negative transcriptional control by four trans-acting MAR (or SIR) loci. We have isolated extragenic suppressors of the mar2-1 mutation which, based on genetic complementation tests, define two additional loci involved in regulating the expression of HML and HMR. A strain with the genotype HMLa MAT alpha HMRa mar2-1 is sterile due to the simultaneous expression of a and alpha information. Two mutants exhibiting an alpha phenotype (which may result from the restoration of MAR/SIR repression) were isolated and genetically characterized. The mutations in these strains: (1) are recessive, (2) are capable of suppressing a mar2-deletion mutation, (3) are unlinked to MAT, (4) complement one another as well as the previously identified sum1-1 mutation, and (5) are not new alleles of the known MAR/SIR loci. We designate these new regulatory loci SUM2 and SUM3 (suppressor of mar). Unlike the sum1-1 mutation, suppression by sum2-1 and sum3-1 is mar2-locus specific. Both sum2-1 and sum3-1 affect the expression of a information at the HM loci. Transcript analysis shows a significant reduction in HMLa and HMRa gene transcription in mar2-1 sum2-1 and mar2-1 sum3-1 cells. Furthermore, we have found genetic evidence to suggest that mar2-1 sum2-1 cells exhibit only partial expression of silent alpha information. We conclude that the SUM2 and SUM3 gene products are required for expression of the HM loci and act downstream of the MAR2 (SIR3) gene function. Possible mechanisms for the action of the SUM gene products are discussed.

Secretion of N-glycosylated human recombinant interleukin-1 alpha in Saccharomyces cerevisiae

We have expressed fragments of the cDNA coding for mature human interleukin-1 alpha (hIL-1 alpha) in Saccharomyces cerevisiae. Mature hIL-1 alpha contains one potential N-linked glycosylation site that is not recognized in mammalian cells. Translational fusions to either one of three yeast signal sequences resulted in secretion of bioactive, N-glycosylated hIL-1 alpha. The extent of glycosylation was significantly reduced using the alpha-factor signal sequence, which itself contains three N-linked glycosylation sites known to be core glycosylated. N-glycosylation has no effect on biological specific activity.