Development and Validation of TaqMan Chemistry Probe-Based Rapid Assay for the Detection of Echinocandin-Resistance in Candida auris

Candida auris is a multidrug-resistant yeast pathogen causing outbreaks in health care facilities worldwide, and the emergence of echinocandin-resistant C. auris is a concern. Currently used Clinical and Laboratory Standards Institute (CLSI) and commercial antifungal susceptibility tests (AFST) are phenotype-based, slow, and not scalable, limiting their effectiveness in the surveillance of echinocandin-resistant C. auris. The urgent need for accurate and rapid methods of assessment of echinocandin resistance cannot be overstated, as this class of antifungal drugs is preferred for patient management. We report the development and validation of a TaqMan chemistry probe-based fluorescence melt curve analysis (FMCA) following asymmetric polymerase chain reaction (PCR) to assess mutations within the hot spot one (HS1) region of FKS1, the gene responsible for encoding 1,3-β-d-glucan synthase that is a target for echinocandins. The assay correctly identified F635C, F635Y, F635del, F635S, S639F or S639Y, S639P, and D642H/R645T mutations. Of these mutations, F635S and D642H/R645T were not involved in echinocandin resistance, while the rest were, as confirmed by AFST. Of 31 clinical cases, the predominant mutation conferring echinocandin resistance was S639F/Y (20 cases) followed by S639P (4 cases), F635del (4 cases), F635Y (2 cases), and F635C (1 case). The FMCA assay was highly specific and did not cross-react with closely and distantly related Candida and other yeast and mold species. Structural modeling of the Fks1 protein, its mutants, and docked conformations of three echinocandin drugs suggest a plausible Fks1 binding orientation for echinocandins. These findings lay the groundwork for future evaluations of additional FKS1 mutations and their impact on the development of drug resistance. The TaqMan chemistry probe-based FMCA would allow rapid, high throughput, and accurate detection of FKS1 mutations conferring echinocandin resistance in C. auris.

Quantitative reverse transcription PCR assay to detect a genetic marker of pyrethroid resistance in Culex mosquitoes

Pyrethroid insecticides are widely used to control mosquitoes that transmit pathogens such as West Nile virus (WNV) to people. Single nucleotide polymorphisms (SNP) in the knockdown resistance locus (kdr) of the voltage gated sodium channel (Vgsc) gene in Culex mosquitoes are associated with knockdown resistance to pyrethroids. RNAseq was used to sequence the coding region of Vgsc for Culex tarsalis Coquillett and Culex erythrothorax Dyar, two WNV vectors. The cDNA sequences were used to develop a quantitative reverse transcriptase PCR assay that detects the L1014F kdr mutation in the Vgsc. Because this locus is conserved, the assay was used successfully in six Culex spp. The resulting Culex RTkdr assay was validated using quantitative PCR and sequencing of PCR products. The accuracy of the Culex RTkdr assay was 99%. The L1014F kdr mutation associated with pyrethroid resistance was more common among Cx. pipiens than other Culex spp. and was more prevalent in mosquitoes collected near farmland. The Culex RTkdr assay takes advantage of the RNA that vector control agencies routinely isolate to assess arbovirus prevalence in mosquitoes. We anticipate that public health and vector control agencies may employ the Culex RTkdr assay to define the geographic distribution of the L1014F kdr mutation in Culex species and improve the monitoring of insecticide resistance that will ultimately contribute to effective control of Culex mosquitoes.

Development of anti-Jk3 associated with silenced Kidd antigen expression and a novel single nucleotide variant of the JK gene

Anti-Jk3 is a rare alloantibody to a high-prevalence antigen primarily seen in individuals of Polynesian descent and is associated with a handful of well-established variants of the SLC14A1 gene. We report a case of the Jk_null phenotype, associated with formation of anti-Jk3, in a patient of non-Polynesian descent. This patient, a 51-year-old woman self-described as of Jamaican and Scottish ancestry, presented to our hospital for oncologic care. The patient's blood sample typed as blood group A, D+. All screening and panel reagent red blood cells showed reactivity, ranging from 2 to 4+; autocontrol and direct antiglobulin test were both negative. Antigen phenotyping revealed Jk(a-b-), leading to suspicion for anti-Jk3, which was subsequently confirmed by our immunohematology reference laboratory. Given her reported familial background, testing of the SLC14A1 gene was performed, revealing that the patient was heterozygous for the single nucleotide variant (SNV) at c.838G>A in exon 8 and therefore carries both JK*01 and JK*02 alleles that encode Jk^a and Jk^b, respectively. However, the patient was found to be heterozygous for several additional SNVs: c.28G>A in exon 3; c.191G>A, c.226G>A, and c.303G>A in exon 4; and c.757T>C in exon 7. The patient's Jk(b-) phenotype can be explained by coinheritance of c.838A with c.191G>A, which defines null allele JK*02N.09. Coinheritance of SNVs c.28G>A and c.838G with rare SNV c.757C that is predicted to cause a non-conservative amino acid change (p.S253P) likely accounts for the complete serologic absence of Jk^a and the ability to form anti-Jk3 in this case. This finding would represent a new JK*01 null allele. This evaluation illustrates the importance of genetic analysis in identifying the factors preventing a high-prevalence antigen from being expressed, particularly when discovered outside of an expected racial or ethnic group.

Anti-Jk3 is a rare alloantibody to a high-prevalence antigen primarily seen in individuals of Polynesian descent and is associated with a handful of well-established variants of the SLC14A1 gene. We report a case of the Jk_null phenotype, associated with formation of anti-Jk3, in a patient of non-Polynesian descent. This patient, a 51-year-old woman self-described as of Jamaican and Scottish ancestry, presented to our hospital for oncologic care. The patient’s blood sample typed as blood group A, D+. All screening and panel reagent red blood cells showed reactivity, ranging from 2 to 4+; autocontrol and direct antiglobulin test were both negative. Antigen phenotyping revealed Jk(a–b–), leading to suspicion for anti-Jk3, which was subsequently confirmed by our immunohematology reference laboratory. Given her reported familial background, testing of the SLC14A1 gene was performed, revealing that the patient was heterozygous for the single nucleotide variant (SNV) at c.838G>A in exon 8 and therefore carries both JK*01 and JK*02 alleles that encode Jk^a and Jk^b, respectively. However, the patient was found to be heterozygous for several additional SNVs: c.28G>A in exon 3; c.191G>A, c.226G>A, and c.303G>A in exon 4; and c.757T>C in exon 7. The patient’s Jk(b–) phenotype can be explained by coinheritance of c.838A with c.191G>A, which defines null allele JK*02N.09. Coinheritance of SNVs c.28G>A and c.838G with rare SNV c.757C that is predicted to cause a non-conservative amino acid change (p.S253P) likely accounts for the complete serologic absence of Jk^a and the ability to form anti-Jk3 in this case. This finding would represent a new JK*01 null allele. This evaluation illustrates the importance of genetic analysis in identifying the factors preventing a high-prevalence antigen from being expressed, particularly when discovered outside of an expected racial or ethnic group.

The lysosomal targeting and intracellular metabolism of trypanosome lytic factor by Trypanosoma brucei brucei

Trypanosome lytic factor (TLF) provides innate protection for humans against infection by the animal pathogen Trypanosoma brucei brucei but not against the agent of human African sleeping sickness, Trypanosoma brucei rhodesiense. TLF exists in two forms, TLF-1 and TLF-2. Prior studies suggested that TLF-1 causes lysosomal disruption and subsequent cell death in T. b. brucei. Here we confirm the lysosomal targeting of TLF-1 by immunolocalization with the trypanosome lysosomal membrane protein p67, and by co-fractionation of radiolabelled TLF-1 with lysosomal enzymes. In addition, pulse-chase studies indicate that TLF-1 is not degraded within the lysosome as compared to the host protein transferrin. In TLF-1 treated cells, transferrin is degraded normally, indicating that lysosomal proteases remain active during the early phase of TLF-1 treatment but fail to degrade TLF-1. Following endocytosis a TLF lipoprotein appears to undergo disulfide bond reduction prior to entering the lysosome. Results presented here indicate that TLF-1 lipoproteins are targeted to the lysosome but are resistant to trypanosome lysosomal proteases.

Transport and trafficking: Toxoplasma as a model for Plasmodium

Like Plasmodium, the protozoan parasite Toxoplasma gondii is a member of the phylum Apicomplexa, and an obligate intracellular pathogen. Unlike Plasmodium, however, Toxoplasma is highly amenable to experimental manipulation in the laboratory. The development of molecular transformation protocols for T. gondii has provided both scientific precedent and practical selectable markers for Plasmodium. Beyond the feasibility of molecular biological experimentation now possible in both systems, the high frequency of stable transformation in Toxoplasma allows this parasite to be used for molecular genetic analysis. The ability to control homologous vs. non-homologous recombination in T. gondii permits gene knockouts/allelic replacements at previously cloned loci, and saturation insertional mutagenesis of the entire parasite genome (and cloning of the tagged loci). T. gondii also exhibits unusual ultrastructural clarity, facilitating cell biological analysis. The accessibility of Toxoplasma as an experimental system allows this parasite to be used as a surrogate for asking many questions that cannot easily be addressed in Plasmodium itself. T. gondii also serves as a model system for genetic exploration of parasite biology and host-parasite interactions. Success stories include: biochemical analysis of antifolate resistance mechanisms; pharmacological studies on the mechanisms of macrolide activity; genetic identification of nucleobase/nucleoside transporters and metabolic pathways; and cell biological characterization of the apicomplexan plastid. As with any model system, not all questions of interest to malariologists can be addressed in Toxoplasma; differentiating between sensible and foolish questions requires familiarity with the biological similarities and differences of these systems.

Survey of current trends in DNA synthesis core facilities

The Nucleic Acids Research Group of the Association of Biomolecular Resource Facilities (ABRF) last surveyed DNA synthesis core facilities in April 1995. Because of the introduction of new technologies and dramatic changes in the market, we sought to update survey information and to determine how academic facilities responded to the challenge presented by commercial counterparts. The online survey was opened in January 1999 by notifying members and subscribers to the ABRF electronic discussion group. The survey consisted of five parts: general facility information, oligonucleotide production profile, oligonucleotide charges, synthesis protocols, and trends in DNA synthesis (including individual comments). All submitted data were anonymously coded. Respondents from DNA synthesis facilities were primarily from the academic category and were established between 1984 and 1991. Typically, a facility provides additional services such as DNA sequencing and has upgraded to electronic ordering. There is stability in staffing profiles for these facilities in that the total number of employees is relatively unchanged, the tenure for staff averages 5.9 years, and experience is extensive. On average, academic facilities annually produce approximately 1/16 the number of oligonucleotides produced by the average commercial facilities, but all facilities report an increase in demand. Charges for standard oligonucleotides from academic facilities are relatively higher than from commercial companies; however, the opposite is true for modified phosphoramidites. Subsidized facilities charge less than nonsubsidized facilities. Synthesis protocols and reagents are standard across the categories. Most facilities offer typical modifications such as biotinylation. Despite the competition by large commercial facilities that have reduced costs dramatically, academic facilities remain a stable entity. Academic facilities enhance the quality of service by focusing on nonstandard oligonucleotides and valuable services such as personal consultations, electronic ordering, and diversifying into other services.

Design strategies and performance of custom DNA sequencing primers

This study surveyed strategies of sequencing primer selection and evaluated primer performance in automated DNA sequencing. We asked participants to relate their preferred primer design strategies to identify primer characteristics that are considered most important in sequencing primer design. The participants preferred primers of 18-24 nucleotides (nt), 39%-58% G + C, a melting temperature (Tm) of 53 degrees-65 degrees C with a 1-2 nt 3' GC clamp, hairpin stems of less than 2-3 bp, homopolymeric runs of less than 4-5 nt, primer dimers of less than 3-4 bp and secondary priming sites of less than 3-4 bp. We provided a 300-bp test sequence and asked participants to submit sequences of 1-3 optimal sequencing primers. Submitted primers ranged from 17-24 nt and largely conformed to the preferred parameters. Submitted primers were distributed across the test sequence, although some sites were disfavored. Surprisingly, approximately 45% of the primers were selected "manually", more than by any software package. Each of 69 submitted and 95 control primers, distributed at 3-bp intervals across the test sequence, were synthesized, purified and tested using a Model 377 PRISM DNA Sequencer with dichlororhodamine dye terminator reagents (dRhodamine dye terminators). Approximately half of the control primers were also tested using rhodamine dye terminator reagents ("old" rhodamine dye terminators). The results indicated that primer physico-chemical characteristics thought to have a strong impact on sequencing performance had surprisingly little effect. Thus, primers with high or low percent G + C or Tm, strong secondary priming scores or long 3' homopolymeric stretches yielded excellent sequences with the dRhodamine dye terminator reagents, although these characteristics had a stronger effect when the old rhodamine reagents were used. The old rhodamine reagents gave sequences with a similar average read length, but the number of errors and ambiguities or "N's" was consistently higher. Moreover, the effects of the primer physico-chemical characteristics were also more evident with the old rhodamine dyes. We conclude that under optimal sequencing conditions with highly pure template and primer, many of the commonly applied primer design parameters are dispensable, particularly when using one of the new generation of sequencing reagents such as the dichlororhodamine dye terminators.

The nuclear envelope serves as an intermediary between the ER and Golgi complex in the intracellular parasite Toxoplasma gondii

Morphological examination of the highly polarized protozoan parasite Toxoplasma gondii suggests that secretory traffic in this organism progresses from the endoplasmic reticulum to the Golgi apparatus using the nuclear envelope as an intermediate compartment. While the endoplasmic reticulum is predominantly located near the basal end of the parasite, the Golgi is invariably adjacent to the apical end of the nucleus, and the space between the Golgi and nuclear envelope is filled with numerous coatomer-coated vesicles. Staining with antiserum raised against recombinant T. gondii beta-COP confirms its association with the apical juxtanuclear region. Perturbation of protein secretion using brefeldin A, microtubule inhibitors or dithiothreitol disrupts the Golgi, causing swelling of the nuclear envelope, particularly at its basal end. Prolonged drug treatment leads to gross distention of the endoplasmic reticulum, filling the basal end of the parasite. Cloning and sequencing of the T. gondii homolog of the chaperonin protein BiP identifies the carboxy-terminal amino acid sequence HDEL as this organism's endoplasmic reticulum-retention signal. Appending the HDEL motif to a recombinant secretory protein (a chimera between the parasite's major surface protein fusion, P30, and the Green Fluorescent Protein) causes this secretory reporter to be retained intracellularly. P30-GFP-HDEL fluorescence was most intense within the nuclear envelope, particularly at the apical end. These data support a model of secretion in which protein traffic from the endoplasmic reticulum to Golgi occurs via the apical end of the nuclear envelope.

Mechanism of resistance of African trypanosomes to cytotoxic human HDL

Trypanosoma brucei brucei, the causative agent of ngana in cattle, is non-infectious to humans because of its sensitivity to the cytolytic activity of normal human serum. The toxin in normal human serum is human haptoglobin-related protein (Hpr) which is found either as an apolipoprotein associated with a minor subclass of high-density lipoprotein (HDL), named trypanosome lytic factor (TLF1), or as an unstable, high-molecular-mass protein complex known as TLF2 (refs 5, 9-12). TLF-mediated lysis of T. b. brucei requires binding, internalization and lysosomal targeting. The human sleeping-sickness trypanosome, Trypanosoma brucei rhodesiense is resistant to TLF. Our studies reveal that resistant trypanosomes fail to endocytose TLF yet continue to bind TLF through cell-surface receptors. On the basis of these results, we conclude that one mechanism of resistance of human sleeping-sickness trypanosomes to human serum is decreased internalization of receptor-bound TLF.

Multi-facility survey of oligonucleotide synthesis and an examination of the performance of unpurified primers in automated DNA sequencing

The purity of 208 crude synthetic 25- and 50-base oligonucleotides synthesized in 71 DNA core facilities was assessed by capillary electrophoresis (CE), and the average coupling efficiency of each synthesis was determined. The median average coupling efficiencies of the 25-mers and 50-mers were 98.9% and 98.7%, respectively, and 85% of the samples exceeded the minimum industry standard of 98% average coupling efficiency. The overall yields estimated by on-line trityl monitors showed poor agreement with the empirically determined yield, and accuracy of the monitors decreased as synthesis efficiency decreased. The performance of the unpurified 25-base oligonucleotides, ranging in purity from 14% to 94%, as primers for automated DNA sequencing was evaluated. Over 85% of these oligonucleotides exhibited an unedited sequencing accuracy of > 97.5% over the 400-base test sequence. Surprisingly, sequencing performance was not strictly related to primer purity, though a marked loss of performance was observed for primers < or = 70% pure (< or = 98.5% coupling efficiency). Thus, the vast majority of the oligonucleotides synthesized by the 71 core facilities participating in this study were of high quality and performed well as sequencing primers without post-synthesis purification or desalting. Finally, our results suggest that an increase in the standard minimum performance specifications of DNA synthesis instruments and reagents from > or = 98% to > or = 98.5% average coupling efficiency, or the development of rapid, inexpensive and efficient methods to detect syntheses below the 98.5% threshold, could obviate post synthesis purification of sequencing primers.

Developmentally regulated sensitivity of Trypanosoma brucei brucei to the cytotoxic effects of human high-density lipoprotein

The bloodstream developmental stages of the protozoan parasite Trypanosoma brucei brucei are lysed by normal human serum. The cytotoxic factor is a minor sub-class of human high-density lipoprotein termed trypanosome lytic factor (TLF). T.b. brucei rapidly develops resistance to TLF when incubated at 26 degrees C under conditions which allow differentiation to the procyclic, insect midgut developmental stage. This in vitro differentiation system allowed us to correlate loss of TLF sensitivity with other parameters of differentiation to the procyclic form. The onset of resistance to TLF occurs within 2 hr after shifting bloodstream forms to differentiation conditions. TLF resistance is correlated with a rapid but transient decrease in protein synthesis by the parasite, is acquired prior to cell division at 26 degrees C, and precedes the loss of variant surface glycoprotein. In addition, we found binding and uptake of TLF by established procyclic trypanosomes was reduced approximately fivefold relative to bloodstream trypanosomes and the TLF binding observed in procyclics was nonspecific. No TLF was bound to the procyclic flagellar pocket membrane and the procyclics failed to endocytose any of the surface-bound TLF. In contrast, bloodstream forms bind TLF via a flagellar pocket-localized protein and bound TLF is taken up by endocytosis. These findings suggest that resistance of procyclic T. b. brucei to TLF-mediated lysis is due to a reduction in the endocytosis of TLF by this developmental stage of the parasite.

Reconstitution of the trypanolytic factor from components of a subspecies of human high-density lipoproteins

Trypanosoma brucei brucei is non-infectious to man due to the sensitivity of these parasites to the lytic activity of normal human serum. Apolipoproteins (apo) have been purified, under non-denaturing conditions, from the subclass of human high-density lipoprotein (HDL), termed trypanosome lytic factor (TLF), which is responsible for the cytotoxicity of human serum to T. b. brucei. The TLF apolipoproteins were purified by anion exchange chromatography in the presence of the nonionic detergent octylglucoside and a reconstitution method was developed which allowed the role of the individual apolipoproteins and different lipids to be assessed. The results suggest that the TLF lipids do not have a direct role in lysis but are necessary for the correct assembly of the lytic HDL particle. Apo A-I, apo L-III and apo L-I contribute to lysis in reconstituted particles but individually they are not cytotoxic. Apo A-II was not required in the reconstituted TLF particle for trypanosome lysis. Formation of a lytic HDL particle required apo L-III suggesting its potential role as a toxin. Thermal inactivation of TLF activity correlated with the amount of denatured apo L-I, indicating that apo L-I was involved in lysis of T. b. brucei by native TLF.

Human high density lipoprotein killing of African trypanosomes

Trypanosoma brucei brucei is noninfectious to humans because of its sensitivity to the cytolytic activity of normal human serum. Biochemical evidence indicates that the active component of human serum is high-density lipoprotein (HDL). Several possible mechanisms have been proposed for the killing of trypanosomes by human HDL, and while a unified model that accounts for all experimental information is lacking, there is substantial evidence that receptor-mediated binding and endocytosis might be required for lysis. Trypanosomes resistant to the lytic effects of human HDL cause human sleeping sickness. The basis for the resistance of these parasites to HDL-mediated killing is unknown. The sensitivity of T. brucei brucei to the cytolytic action of human HDL is developmentally regulated and resistance correlates with life-cycle specific changes in macromolecular uptake by the organism.

Endocytosis of a cytotoxic human high density lipoprotein results in disruption of acidic intracellular vesicles and subsequent killing of African trypanosomes

The host range of Trypanosoma brucei brucei is restricted by the cytolytic effects of human serum high-density lipoprotein (HDL). The lytic activity is caused by a minor subclass of human serum HDL called trypanosome lytic factor (TLF). TLF binds in the flagellar pocket to specific TLF-binding sites. Internalization and localization of TLF to a population of endocytic vesicles, and ultimately large lysosome-like vesicles, precedes lysis of T. b. brucei. The membranes of these large vesicles are disrupted by the accumulation of TLF particles. Inhibitor studies with lysosomotropic amines have shown these large vesicles to be acidic in nature and that prevention of their rupture spares the cells from TLF-mediated lysis. Furthermore, leupeptin inhibition suggests that a thioprotease may be involved in the mechanism of TLF-mediated lysis of T. b. brucei. Based on these results, we propose a lytic mechanism involving cell surface binding, endocytosis and lysosomal targeting. This is followed by lysosomal disruption and subsequent autodigestion of the cell.

Sodium cotransport proteins

Significant advances have been made in elucidating the structure of Na+ cotransport proteins. Some fifteen of these low-abundance proteins have been cloned, sequenced and functionally expressed. They are members of the 12 membrane-spanning superfamily and they segregate into two groups, the Na+/glucose (SGLT1) and Na+/Cl-/GABA (GAT-1) families. SGLT1 transporters are expressed in bacteria and animal cells, while GAT-1 transporters are mostly expressed in the brain. None have yet been found in plants.

Timing of synthesis and cellular localization of two conidiation-specific proteins of Neurospora crassa

The process of conidiation in Neurospora crassa consists of a series of distinct developmental stages culminating in the formation of multinucleate asexual spores called macroconidia. Immunoblotting techniques were used to study the timing of synthesis and cellular localization of CON10 and CON13, the products of two genes that are expressed during conidiation but not during mycelial growth. Both proteins first appear about 8 hr into conidiation; CON10 disappears between 2 and 4 hr after germination. Within conidiating cultures, CON10 and CON13 proteins are localized in conidiophores, with little or no protein present in the underlying mycelium. Immunofluorescence analyses show that CON10 is evenly distributed throughout the cytoplasm of macroconidia. Synthesis of CON10 and CON13 occurs at a time when their specifying mRNAs first appear (Hager and Yanofsky, Gene 96, 153-159, 1990; Sachs and Yanofsky, Dev. Biol 148, 117-128, 1991), suggesting that regulation of synthesis is predominantly transcriptional.

Genes expressed during conidiation in Neurospora crassa: molecular characterization of con-13

Asexual development in Neurospora crassa proceeds through a series of discrete morphological stages that culminate in the production of dormant spores called conidia. Changes in the pattern of gene expression parallel the morphological transformations associated with conidiation. As a prerequisite to the analysis of developmental gene expression in N. crassa, several genes of unknown function that are preferentially expressed during conidiation were isolated [Berlin and Yanofsky, Mol. Cell. Biol. 5 (1985) 849-855]. The molecular structure and nucleotide sequence of one of these genes, designated con-13, is presented. The con-13 gene specifies a relatively rare 1.35-kb message which is first detected about 8 h following the induction of conidiation. Sequence analysis of both cDNA and genomic clones indicates that the con-13 gene consists of three exons divided by two small introns. It encodes a polypeptide of 340 amino acid residues (37.1 kDa). The Con-13 protein is weakly acidic and hydrophilic. A comparison of the regions upstream from the con-8, con-10, and con-13 genes revealed several short sequence motifs which may be important in developmental gene regulation.

Biosynthesis of the plasma membrane H+-ATPase of Neurospora crassa

The plasma membrane H+-ATPase of Neurospora is a 100-kDa integral membrane protein which appears, on the basis of hydropathy analysis of its amino acid sequence, to span the lipid bilayer at least eight times. To investigate the assembly and processing of the ATPase, a full-length cDNA has been constructed for use in in vitro transcription and translation experiments. Comparison of three different forms of the ATPase (nascent protein, nascent protein cotranslationally inserted into membranes, and mature protein) revealed no difference in electrophoretic mobility. Furthermore, the nascent and mature forms gave identical peptide patterns after partial proteolysis with Staphylococcus aureus V8 protease, suggesting that the ATPase does not contain an NH2-terminal signal peptide which is cleaved upon membrane insertion. Consistent with this interpretation, the NH2-terminal peptide has been purified from a tryptic digest of the ATPase and found to lack only the initiator methionine residue; the penultimate alanine is acetylated based on analysis by fast atom bombardment mass spectroscopy. Although the ATPase contains one potential site of N-linked glycosylation, its electrophoretic mobility was unchanged following digestion with endoglycosidase H and it did not incorporate [3H]mannose or bind concanavalin A. Thus, the Neurospora plasma membrane-ATPase appears to undergo minimal post-translational processing, and its membrane insertion is probably mediated by internal sequences.

Location of a dicyclohexylcarbodiimide-reactive glutamate residue in the Neurospora crassa plasma membrane H+-ATPase

The proton pump (H+-ATPase) found in the plasma membrane of the fungus Neurospora crassa is inactivated by dicyclohexylcarbodiimide (DCCD). Kinetic and labeling experiments have suggested that inactivation at 0 degrees C results from the covalent attachment of DCCD to a single site in the Mr = 100,000 catalytic subunit (Sussman, M. R., and Slayman, C. W. (1983) J. Biol. Chem. 258, 1839-1843). In the present study, when [14C]DCCD-labeled enzyme was treated with the cleavage reagent, N-bromosuccinimide, a single major radioactive peptide fragment migrating at about Mr = 5,300 on sodium dodecyl sulfate-polyacrylamide gel electrophoresis was produced. The fragment was coupled to glass beads and partially sequenced by automated solid-phase Edman degradation at the amino terminus and at an internal tryptic cleavage site. By comparison to the DNA-derived amino acid sequence for the entire Mr = 100,000 polypeptide (Hager, K., and Slayman, C. W. (1986) Proc. Natl. Acad. Sci. U. S. A. 83, 7693-7697), the fragment has been identified as arising by cleavage at tyrosine 100 and tryptophan 141. Covalently incorporated [14C]DCCD was released at a position corresponding to glutamate 129. The DCCD-reactive glutamate is located in the middle of the first of eight predicted transmembrane sequences. When the sequence surrounding the DCCD site is compared to that surrounding the DCCD-reactive residue of two other proton pumps, the F0F1-ATPase and cytochrome c oxidase, no homology is apparent apart from an abundance of hydrophobic amino acids.

Amino acid sequence of the plasma membrane ATPase of Neurospora crassa: deduction from genomic and cDNA sequences

The plasma membrane of Neurospora crassa contains an electrogenic H+-ATPase (EC 3.6.1.35), for which we have isolated and sequenced both genomic and cDNA clones. The ATPase gene is interrupted by four small introns (58-124 base pairs). It encodes a protein of 920 amino acids (Mr, 99,886) possessing as many as eight transmembrane segments. The Neurospora ATPase shows significant amino acid sequence homology with the Na+,K+- and Ca2+-transporting ATPases of animal cells, particularly in regions that appear to be involved in ATP binding and hydrolysis.