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R C Patel

Publications and source records attributed to R C Patel.

At least 19 recordsLinked to original sources
PubMed2000

PACT, a stress-modulated cellular activator of interferon-induced double-stranded RNA-activated protein kinase, PKR.

The interferon (IFN)-induced, double-stranded (ds)RNA-activated serine-threonine protein kinase, PKR, is a key mediator of the antiviral activities of IFNs. In addition, PKR activity is also involved in regulation of cell proliferation, apoptosis, and signal transduction. In virally infected cells, dsRNA has been shown to bind and activate PKR kinase function. Implication of PKR activity in normal cellular processes has invoked activators other than dsRNA because RNAs with perfectly duplexed regions of sufficient length that are able to activate PKR are absent in cellular RNAs. We have recently reported cloning of PACT, a novel protein activator of PKR. PACT heterodimerizes with PKR and activates it by direct protein-protein interaction. Overexpression of PACT in mammalian cells leads to phosphorylation of the alpha subunit of the eukaryotic initiation factor 2 (eIF2alpha), the cellular substrate for PKR, and leads to inhibition of protein synthesis. Here, we present evidence that endogenous PACT acts as a protein activator of PKR in response to diverse stress signals such as serum starvation, and peroxide or arsenite treatment. Following exposure of cells to these stress agents, PACT is phosphorylated and associates with PKR with increased affinity. PACT-mediated activation of PKR leads to enhanced eIF2alpha phosphorylation followed by apoptosis. Based on the results presented here, we propose that PACT is a novel stress-modulated physiological activator of PKR.

3T3 Cells↗
PubMed2000

Receptors for dopamine and somatostatin: formation of hetero-oligomers with enhanced functional activity.

Somatostatin and dopamine are two major neurotransmitter systems that share a number of structural and functional characteristics. Somatostatin receptors and dopamine receptors are colocalized in neuronal subgroups, and somatostatin is involved in modulating dopamine-mediated control of motor activity. However, the molecular basis for such interaction between the two systems is unclear. Here, we show that dopamine receptor D2R and somatostatin receptor SSTR5 interact physically through hetero-oligomerization to create a novel receptor with enhanced functional activity. Our results provide evidence that receptors from different G protein (heterotrimeric guanine nucleotide binding protein)-coupled receptor families interact through oligomerization. Such direct intramembrane association defines a new level of molecular crosstalk between related G protein-coupled receptor subfamilies.

Animals↗
PubMed2000

Subtypes of the somatostatin receptor assemble as functional homo- and heterodimers.

The existence of receptor dimers has been proposed for several G protein-coupled receptors. However, the question of whether G protein-coupled receptor dimers are necessary for activating or modulating normal receptor function is unclear. We address this question with somatostatin receptors (SSTRs) of which there are five distinct subtypes. By using transfected mutant and wild type receptors, as well as endogenous receptors, we provide pharmacological, biochemical, and physical evidence, based on fluorescence resonance energy transfer analysis, that activation by ligand induces SSTR dimerization, both homo- and heterodimerization with other members of the SSTR family, and that dimerization alters the functional properties of the receptor such as ligand binding affinity and agonist-induced receptor internalization and up-regulation. Double label confocal fluorescence microscopy showed that when SSTR1 and SSTR5 subtypes were coexpressed in Chinese hamster ovary-K1 cells and treated with agonist they underwent internalization and were colocalized in cytoplasmic vesicles. SSTR5 formed heterodimers with SSTR1 but not with SSTR4 suggesting that heterodimerization is a specific process that is restricted to some but not all receptor subtype combinations. Direct protein interaction between different members of the SSTR subfamily defines a new level of molecular cross-talk between subtypes of the SSTR and possibly related receptor families.

Dimerization↗
PubMed2000

Salivary histatin 5 and human neutrophil defensin 1 kill Candida albicans via shared pathways.

Salivary histatins are a family of basic histidine-rich proteins in which therapeutic potential as drugs against oral candidiasis is apparent, considering their potent in vitro antifungal activity and lack of toxicity to humans. Histatin 5 (Hst 5) kills the fungal pathogen Candida albicans via a mechanism that involves binding to specific sites on the yeast cell membrane and subsequent release of cellular ATP in the absence of cytolysis. We explored the killing pathway activated by Hst 5 and compared it to those activated by other antifungal agents. The candidacidal activity of human neutrophil defensin 1 (HNP-1) shared very similar features to Hst 5 cytotoxic action with respect to active concentrations and magnitude of induction of nonlytic ATP efflux, depletion of intracellular ATP pools, and inhibitor profile. Hst 5 and HNP-1 are basic proteins of about 3 kDa; however, they have unique primary sequences and solution structures that cannot explain how these two molecules act so similarly on C. albicans to induce cell death. Our finding that HNP-1 prevented Hst 5 binding to the candidal Hst 5 binding protein suggests that the basis for the overlapping actions of these two naturally occurring antimicrobial proteins may involve interactions with shared yeast components.

Adenosine Triphosphate↗
PubMed1999

DRBP76, a double-stranded RNA-binding nuclear protein, is phosphorylated by the interferon-induced protein kinase, PKR.

The interferon-induced double-stranded RNA-activated protein kinase PKR is the prototype of a class of double-stranded (dsRNA)-binding proteins (DRBPs) which share a dsRNA-binding motif conserved from Drosophila to humans. Here we report the purification of DRBP76, a new human member of this class of proteins. Sequence from the amino terminus of DRBP76 matched that of the M phase-specific protein, MPP4. DRBP76 was also cloned by the yeast two-hybrid screening of a cDNA library using a mutant PKR as bait. Analysis of the cDNA sequence revealed that it is the full-length version of MPP4, has a bipartite nuclear localization signal, two motifs that can mediate interactions with both dsRNA and PKR, five epitopes for potential M phase-specific phosphorylation, two potential sites for phosphorylation by cyclin-dependent kinases, a RG2 motif present in many RNA-binding proteins and predicts a protein of 76 kDa. DsRNA and PKR interactions of DRBP76 were confirmed by analysis of in vitro translated and purified native proteins. Cellular expression of an epitope-tagged DRBP76 demonstrated its nuclear localization, and its co-immunoprecipitation with PKR demonstrated that the two proteins interact in vivo. Finally, purified DRBP76 was shown to be a substrate of PKR in vitro, indicating that this protein's cellular activities may be regulated by PKR-mediated phosphorylation.

Amino Acid Sequence↗
PubMed1999

Localization of Niemann-Pick C1 protein in astrocytes: implications for neuronal degeneration in Niemann- Pick type C disease.

Niemann-Pick type C disease (NP-C) is an inherited neurovisceral lipid storage disorder characterized by progressive neurodegeneration. Most cases of NP-C result from inactivating mutations of NPC1, a recently identified member of a family of genes encoding membrane-bound proteins containing putative sterol sensing domains. By using a specific antipeptide antibody to human NPC1, we have here investigated the cellular and subcellular localization and regulation of NPC1. By light and electron microscopic immunocytochemistry of monkey brain, NPC1 was expressed predominantly in perisynaptic astrocytic glial processes. At a subcellular level, NPC1 localized to vesicles with the morphological characteristics of lysosomes and to sites near the plasma membrane. Analysis of the temporal and spatial pattern of neurodegeneration in the NP-C mouse, a spontaneous mutant model of human NP-C, by amino-cupric-silver staining, showed that the terminal fields of axons and dendrites are the earliest sites of degeneration that occur well before the appearance of a neurological phenotype. Western blots of cultured human fibroblasts and monkey brain homogenates revealed NPC1 as a 165-kDa protein. NPC1 levels in cultured fibroblasts were unchanged by incubation with low density lipoproteins or oxysterols but were increased 2- to 3-fold by the drugs progesterone and U-18666A, which block cholesterol transport out of lysosomes, and by the lysosomotropic agent NH4Cl. These studies show that NPC1 in brain is predominantly a glial protein present in astrocytic processes closely associated with nerve terminals, the earliest site of degeneration in NP-C. Given the vesicular localization of NPC1 and its proposed role in mediating retroendocytic trafficking of cholesterol and other lysosomal cargo, these results suggest that disruption of NPC1-mediated vesicular trafficking in astrocytes may be linked to neuronal degeneration in NP-C.

Amino Acid Sequence↗
PubMed1998

Retinol and retinoic acid bind to a surface cleft in bovine beta-lactoglobulin: a method of binding site determination using fluorescence resonance energy transfer.

Two potential ligand binding sites in the lipocalin beta-lactoglobulin have been postulated for small hydrophobic molecules such as retinol or retinoic acid. An agreement on one of the two alternatives, an interior cavity or a surface cleft, however, has not been achieved. In order to discriminate between these two possibilities, we measured the efficiency of fluorescence resonance energy transfer between the two intrinsic Trp-residues of beta-lactoglobulin and the ligands retinol, retinoic acid and bis-ANS. Using the crystallographic coordinates of beta-lactoglobulin, this efficiency could be accurately computed for both the interior cavity and the surface cleft as ligand binding sites. For the surface cleft, the theoretical value was found to be in excellent agreement with the measured value, whereas for the interior cavity any reasonable agreement would require a dramatic ligand-induced conformational change that can be ruled out due to the protein's known structural stability. Our conclusion that these ligands bind to the surface pocket rather than the interior cavity was further confirmed by competitive binding studies.

Animals↗
PubMed1998

PACT, a protein activator of the interferon-induced protein kinase, PKR.

PKR, a latent protein kinase, mediates the antiviral actions of interferon. It is also involved in cellular signal transduction, apoptosis, growth regulation and differentiation. Although in virus-infected cells, viral double-stranded (ds) RNA can serve as a PKR activator, cellular activators have remained obscure. Here, we report the cloning of PACT, a cellular protein activator of PKR. PACT heterodimerized with PKR and activated it in vitro in the absence of dsRNA. In mammalian cells, overexpression of PACT caused PKR activation and, in yeast, co-expression of PACT enhanced the anti-growth effect of PKR. Thus, PACT has the hallmarks of a direct activator of PKR.

Amino Acid Sequence↗
PubMed1998

Cellular cholesterol storage in the Niemann-Pick disease type C mouse is associated with increased expression and defective processing of apolipoprotein D.

Apolipoprotein D (apoD), a member of the lipocalin superfamily of ligand transporters, has been implicated in the transport of several small hydrophobic molecules including sterols and steroid hormones. We have previously established that apoD is a secreted protein from cultured mouse astrocytes and that treatment with the oxysterol 25-hydroxycholesterol markedly stimulates apoD release. Here, we have investigated expression and cellular processing of apoD in the Niemann-Pick type C (NPC) mouse, an animal model of human NPC, which is a genetic disorder affecting cellular cholesterol transport. NPC is phenotypically characterized by symptoms of chronic progressive neurodegeneration. ApoD gene expression was up-regulated in cultured NPC astrocytes and in NPC brain. ApoD protein levels were also increased in NPC brain with up to 30-fold higher apoD content in the NPC cerebellum compared with control mice. Subcellular fractionation of NPC brain homogenates revealed that most of the apoD was associated with the myelin fraction. ApoD was found to be a secreted protein from cultured normal astrocytes and treatment with the oxysterol, 25-hydroxycholesterol, markedly stimulated apoD release (by five- to 10-fold). By contrast, secretion of apoD from NPC astrocytes was markedly reduced and could not be stimulated by oxysterol treatment. Secretion of apoE, another apolipoprotein normally produced by astrocytes, was similar in NPC and control cells. Furthermore, apoE secretion was not potentiated by oxysterol treatment in either cell type. Plasma levels of apoD were sixfold higher in NPC, whereas hepatic levels were substantially reduced compared with controls, possibly reflecting reduced hepatic clearance of the circulating protein. These results reveal hitherto unrecognized defects in apoD metabolism in NPC that appear to be linked to the known defects in cholesterol homeostasis in this disorder.

Animals↗
PubMed1998

Requirement of PKR dimerization mediated by specific hydrophobic residues for its activation by double-stranded RNA and its antigrowth effects in yeast.

The roles of protein dimerization and double-stranded RNA (dsRNA) binding in the biochemical and cellular activities of PKR, the dsRNA-dependent protein kinase, were investigated. We have previously shown that both properties of the protein are mediated by the same domain. Here we show that dimerization is mediated by hydrophobic residues present on one side of an amphipathic alpha-helical structure within this domain. Appropriate substitution mutations of residues on that side produced mutants with increased or decreased dimerization activities. Using these mutants, we demonstrated that dimerization is not essential for dsRNA binding. However, enhancing dimerization artificially, by providing an extraneous dimerization domain, increased dsRNA binding of both wild-type and mutant proteins. In vitro, the dimerization-defective mutants could not be activated by dsRNA but were activated normally by heparin. In Saccharomyces cerevisiae, unlike wild-type PKR, these mutants could not inhibit cell growth and the dsRNA-binding domain of the dimerization-defective mutants could not prevent the antigrowth effect of wild-type PKR. These results demonstrate the biological importance of the dimerization properties of PKR.

Animals↗
PubMed1997

Probing the structure of the ligand binding cavity of lipocalins by fluorescence spectroscopy.

The lipocalin superfamily constitutes a phylogenetically conserved group of more than 40 proteins that function in the binding and transport of a variety of physiologically important ligands. Members of this family subserve diverse functions as carriers of retinoids (retinol binding protein), odorants (odorant binding proteins), chromophores (insecticyanin, INS), pheromones (aphrodisin) and sterols (apolipoprotein D, apoD). Despite the pivotal importance of the ligand binding function of these proteins, a suitable approach for characterizing the molecular determinants of such binding has not been available. In studies using three homogeneously purified lipocalins INS, beta-lactoglobulin (BLG) and human apoD, we find that the fluorescence reporter BIS (1,1'-bi(4-anilino) naphthalene-5,5'-disulfonic acid) is an ideal candidate for use in rapid kinetic experiments and in fluorescence resonance energy transfer (FRET). These methods require only small amounts of reagents and yield molecular coordinates of the ligand binding cavity of lipocalins in solution that are in remarkably close agreement to those obtained from crystallographic work with solids. Extremely fast ligand binding dynamics is indicated.

Animals↗
PubMed1996

Specific mutations near the amino terminus of double-stranded RNA-dependent protein kinase (PKR) differentially affect its double-stranded RNA binding and dimerization properties.

The amino-terminal region of the double-stranded (ds) RNA-dependent protein kinase, PKR, has been shown to mediate both dsRNA binding and protein dimerization. To critically examine if PKR dimerization is dependent on dsRNA binding, we generated a series of mutants that are incapable of binding dsRNA. Some, but not all, of these mutants retained the ability to dimerize, as shown by a two-hybrid transcriptional activation assay in vivo and a chemical cross-linking assay in vitro. These mutants were used further to demonstrate that the translational inhibitory activity of PKR in vivo requires dsRNA binding; PKR mutants that dimerized but did not bind dsRNA could not inhibit the translation of a transfected reporter gene.

Amino Acid Sequence↗
PubMed1996

Application of the Roche Amplicor Mycobacterium tuberculosis (PCR) test to specimens other than respiratory secretions.

The ability of the Roche AMPLICOR Mycobacterium tuberculosis (MTB) test to detect M. tuberculosis in specimens other than respiratory secretions was evaluated. A total of 249 specimens from 219 patients were tested. Of these, 12 specimens grew isolates of the M. tuberculosis complex and four grew isolates of the M. avium complex. The AMPLICOR MTB test was positive for 10 of the 12 specimens which grew M. tuberculosis and for three specimens which were culture negative. Two of the latter specimens were from patients with a clinical diagnosis of tuberculosis and with multiple sputum specimens which grew M. tuberculosis. Four specimens grew M. avium complex isolates, and all yielded negative AMPLICOR MTB test results. The sensitivity, specificity, and positive and negative predictive values for the AMPLICOR MTB test were 85.7%, 99.5%, 92.3%, and 99.1%, respectively. Our data indicate that the AMPLICOR MTB test will permit the rapid detection of M. tuberculosis in specimens other than respiratory secretions.

Biopsy↗
PubMed1996

Cardiac involvement in HIV infection.

Cardiac involvement in HIV infection was previously believed to be an unusual manifestation of the disease, but is now being described with increasing frequency. It may be a well-characterized cardiac disease occurring coincidentally in an AIDS patient, a complication of AIDS or its treatment, or possibly a direct result of HIV infection of the heart. In this article, the authors describe the emerging patterns of heart and vascular diseases in HIV-infected patients, pathogenic mechanisms, and implications for treatment.

AIDS-Related Opportunistic Infections↗
PubMed1995

The interferon-inducible double-stranded RNA-activated protein kinase self-associates in vitro and in vivo.

The interferon-inducible double-stranded (ds) RNA-activated protein kinase (PKR) exhibits antiviral, anticellular, and antitumor activities. The mechanisms of its enzymatic activation by autophosphorylation and of the observed transdominant inhibitory phenotype of enzymatically inactive mutants have invoked PKR dimerization. Here we present direct evidence in support of PKR-PKR interaction. We show that radiolabeled PKR can specifically interact with matrix-bound unlabeled PKR in the absence of dsRNA. The self-association activity resides, in part, in the N-terminal region of 170 residues, which also constitutes the dsRNA-binding domain (DRBD). DRBD can bind to matrix-bound PKR or to matrix-bound DRBD. Dimerization of DRBD was directly demonstrated by chemical crosslinking. Affinity chromatography and electrophoretic mobility supershift assays demonstrated that mutants that fail to bind dsRNA can still exhibit protein-protein interaction. The PKR-PKR interaction could also be observed in a two-hybrid transcriptional activation assay in mammalian cells and consequently is likely to be an important feature of PKR activity in vivo.

Biopolymers↗
PubMed1995

Astrocytes synthesize and secrete the lipophilic ligand carrier apolipoprotein D.

Expression of the lipophilic ligand transporter, apolipoprotein D (apoD) by primary astrocyte cultures derived from neonatal mouse brain was investigated. Western blot analysis of cell lysates and media showed that apoD is constitutively secreted by astrocytes with little intracellular storage. The secreted apoD floated primarily at density 1.063-1.21 g ml-1 upon sequential ultracentrifugation indicating its association with lipids. Treatment of astrocytes with the carboxylic ionophore, monensin, resulted in intracellular retention and decreased secretion of apoD that was of slightly reduced M(r). Progesterone, a steroid hormone that binds to apoD with high affinity (10(-6) mol l-1) and the oxysterol, 25-hydroxycholesterol which is a potent regulator of cellular cholesterol homeostasis in mammalian cells, differentially stimulated apoD, but not apoE secretion. These results show that astrocytes synthesize and constitutively secrete apoD and suggest a physiologic role for this lipocalin in cholesterol metabolism in the nervous system.

Animals↗
PubMed1995

Activation of interferon-inducible 2'-5' oligoadenylate synthetase by adenoviral VAI RNA.

2'-5' oligoadenylate (2-5(A)) synthetase and protein kinase, RNA activated (PKR) are the only two known enzymes that bind double-stranded RNA (dsRNA) and get activated by it. We have previously identified their dsRNA binding domains, which do not have any sequence homology. Here, we report a profound difference between the two enzymes with respect to the structural features of the dsRNA that are required for their activation. The adenoviral virus-associated type I (VAI) RNA cannot activate PKR, although it binds to the protein and thereby prevents its activation by authentic dsRNA. In contrast, we observed that VAI RNA can both bind and activate 2-5(A) synthetase. Mutations in VAI RNA, which removed occasional mismatches present in its double-stranded stems, markedly enhanced its 2-5(A) synthetase-activating capacity. These mutants, however, are incapable of activating PKR. Other mutations, which disrupted the structure of the central stem-loop region of the VAI RNA, reduced its ability to activate 2-5(A) synthetase. These debilitated mutants could bind to the synthetase protein, although they fail to bind to PKR.

2',5'-Oligoadenylate Synthetase↗