Characterisation of complexes formed by parasite proteins exported into the host cell compartment of Plasmodium falciparum infected red blood cells

During its intraerythrocytic life cycle, the human malaria parasite Plasmodium falciparum supplements its nutritional requirements by scavenging substrates from the plasma through the new permeability pathways (NPPs) installed in the red blood cell (RBC) membrane. Parasite proteins of the RhopH complex: CLAG3, RhopH2, RhopH3, have been implicated in NPP activity. Here, we studied 13 exported proteins previously hypothesised to interact with RhopH2, to study their potential contribution to the function of NPPs. NPP activity assays revealed that the 13 proteins do not appear to be individually important for NPP function, as conditional knockdown of these proteins had no effect on sorbitol uptake. Intriguingly, reciprocal immunoprecipitation assays showed that five of the 13 proteins interact with all members of the RhopH complex, with PF3D7_1401200 showing the strongest association. Mass spectrometry‐based proteomics further identified new protein complexes; a cytoskeletal complex and a Maurer's clefts/J‐dot complex, which overall helps clarify protein–protein interactions within the infected RBC (iRBC) and is suggestive of the potential trafficking route of the RhopH complex itself to the RBC membrane.

Proteomic Analysis of Antigen 60 Complex of M. bovis Bacillus Calmette-Guérin Reveals Presence of Extracellular Vesicle Proteins and Predicted Functional Interactions

Tuberculosis (TB) is ranked among the top 10 causes of death worldwide. New biomarker-based serodiagnostics and vaccines are unmet needs stalling disease control. Antigen 60 (A60) is a thermostable mycobacterial complex typically purified from Bacillus Calmette-Guérin (BCG) vaccine. A60 was historically evaluated for TB serodiagnostic and vaccine potential with variable findings. Despite containing immunogenic proteins, A60 has yet to be proteomically characterized. Here, commercial A60 was (1) trypsin-digested in-solution, analyzed by LC-MS/MS, searched against M. tuberculosis H37Rv and M. bovis BCG Uniprot databases; (2) analyzed using STRING to predict protein-protein interactions; and (3) probed with anti-TB monoclonal antibodies and patient immunoglobulin G (IgG) on Western blot to evaluate antigenicity. We detected 778 proteins in two A60 samples (440 proteins shared), including DnaK, LprG, LpqH, and GroEL1/2, reportedly present in mycobacterial extracellular vesicles (EV). Of these, 107 were also reported in EVs of M. tuberculosis, and 27 key proteins had significant protein-protein interaction, with clustering for chaperonins, ribosomal proteins, and proteins for ligand transport (LpqH and LprG). On Western blot, 7/8 TB and 1/8 non-TB sera samples had reactivity against 37-50 kDa proteins, while LpqH, GroEL2, and PstS1 were strongly detected. In conclusion, A60 comprises numerous proteins, including EV proteins, with predicted biological interactions, which may have implications on biomarker and vaccine development.

Functional Conservation of the AMA1 Host-Cell Invasion Ligand Between P. falciparum and P. vivax: A Novel Platform to Accelerate Vaccine and Drug Development

Plasmodium vivax and P. falciparum malaria species have diverged significantly in receptor-ligand interactions and host-cell invasion. One protein common to both is the merozoite invasion ligand AMA1. While the general structure of AMA1 is similar between species, their sequences are divergent. Surprisingly, it was possible to genetically replace PfAMA1 with PvAMA1 in P. falciparum parasites. PvAMA1 complemented PfAMA1 function and supported invasion of erythrocytes by P. falciparum. Genetically modified P. falciparum expressing PvAMA1 evaded the invasion inhibitory effects of antibodies to PfAMA1, demonstrating species specificity of functional antibodies. We generated antibodies to recombinant PvAMA1 that effectively inhibited invasion, confirming the function of PvAMA1 in genetically modified parasites. Results indicate significant molecular flexibility in AMA1 enabling conserved function despite substantial sequence divergence across species. This provides powerful new tools to quantify the inhibitory activities of antibodies or drugs targeting PvAMA1, opening new opportunities for vaccine and therapeutic development against P. vivax.

The malaria PTEX component PTEX88 interacts most closely with HSP101 at the host-parasite interface

The pathogenic nature of malaria infections is due in part to the export of hundreds of effector proteins that actively remodel the host erythrocyte. The Plasmodium translocon of exported proteins (PTEX) has been shown to facilitate the trafficking of proteins into the host cell, a process that is essential for the survival of the parasite. The role of the auxiliary PTEX component PTEX88 remains unclear, as previous attempts to elucidate its function through reverse genetic approaches showed that in contrast to the core components PTEX150 and HSP101, knockdown of PTEX88 did not give rise to an export phenotype. Here, we have used biochemical approaches to understand how PTEX88 assembles within the translocation machinery. Proteomic analysis of the PTEX88 interactome showed that PTEX88 interacts closely with HSP101 but has a weaker affinity with the other core constituents of PTEX. PTEX88 was also found to associate with other PV-resident proteins, including chaperones and members of the exported protein-interacting complex that interacts with the major virulence factor PfEMP1, the latter contributing to cytoadherence and parasite virulence. Despite being expressed for the duration of the blood-stage life cycle, PTEX88 was only discretely observed at the parasitophorous vacuole membrane during ring stages and could not always be detected in the major high molecular weight complex that contains the other core components of PTEX, suggesting that its interaction with the PTEX complex may be dynamic. Together, these data have enabled the generation of an updated model of PTEX that now includes how PTEX88 assembles within the complex.

The exported chaperone Hsp70-x supports virulence functions for Plasmodium falciparum blood stage parasites

Malaria is caused by five different Plasmodium spp. in humans each of which modifies the host erythrocyte to survive and replicate. The two main causes of malaria, P. falciparum and P. vivax, differ in their ability to cause severe disease, mainly due to differences in the cytoadhesion of infected erythrocytes (IE) in the microvasculature. Cytoadhesion of P. falciparum in the brain leads to a large number of deaths each year and is a consequence of exported parasite proteins, some of which modify the erythrocyte cytoskeleton while others such as PfEMP1 project onto the erythrocyte surface where they bind to endothelial cells. Here we investigate the effects of knocking out an exported Hsp70-type chaperone termed Hsp70-x that is present in P. falciparum but not P. vivax. Although the growth of Δhsp70-x parasites was unaffected, the export of PfEMP1 cytoadherence proteins was delayed and Δhsp70-x IE had reduced adhesion. The Δhsp70-x IE were also more rigid than wild-type controls indicating changes in the way the parasites modified their host erythrocyte. To investigate the cause of this, transcriptional and translational changes in exported and chaperone proteins were monitored and some changes were observed. We propose that PfHsp70-x is not essential for survival in vitro, but may be required for the efficient export and functioning of some P. falciparum exported proteins.

Plasmodium falciparum parasites deploy RhopH2 into the host erythrocyte to obtain nutrients, grow and replicate

Plasmodium falciparum parasites, the causative agents of malaria, modify their host erythrocyte to render them permeable to supplementary nutrient uptake from the plasma and for removal of toxic waste. Here we investigate the contribution of the rhoptry protein RhopH2, in the formation of new permeability pathways (NPPs) in Plasmodium-infected erythrocytes. We show RhopH2 interacts with RhopH1, RhopH3, the erythrocyte cytoskeleton and exported proteins involved in host cell remodeling. Knockdown of RhopH2 expression in cycle one leads to a depletion of essential vitamins and cofactors and decreased de novo synthesis of pyrimidines in cycle two. There is also a significant impact on parasite growth, replication and transition into cycle three. The uptake of solutes that use NPPs to enter erythrocytes is also reduced upon RhopH2 knockdown. These findings provide direct genetic support for the contribution of the RhopH complex in NPP activity and highlight the importance of NPPs to parasite survival.

Differing rates of antibody acquisition to merozoite antigens in malaria: implications for immunity and surveillance

Antibodies play a key role in acquired human immunity to Plasmodium falciparum (Pf) malaria and target merozoites to reduce or prevent blood-stage replication and the development of disease. Merozoites present a complex array of antigens to the immune system, and currently, there is only a partial understanding of the targets of protective antibodies and how responses to different antigens are acquired and boosted. We hypothesized that there would be differences in the rate of acquisition of antibodies to different antigens and how well they are boosted by infection, which impacts the acquisition of immunity. We examined responses to a range of merozoite antigens in 2 different cohorts of children and adults with different age structures and levels of malaria exposure. Overall, antibodies were associated with age, exposure, and active infection, and the repertoire of responses increased with age and active infection. However, rates of antibody acquisition varied between antigens and different regions within an antigen following exposure to malaria, supporting our hypothesis. Antigen-specific responses could be broadly classified into early response types in which antibodies were acquired early in childhood exposure and late response types that appear to require substantially more exposure for the development of substantial levels. We identified antigen-specific responses that were effectively boosted after recent infection, whereas other responses were not. These findings advance our understanding of the acquisition of human immunity to malaria and are relevant to the development of malaria vaccines targeting merozoite antigens and the selection of antigens for use in malaria surveillance.

Proteomic analysis reveals novel proteins associated with the Plasmodium protein exporter PTEX and a loss of complex stability upon truncation of the core PTEX component, PTEX150

The Plasmodium translocon for exported proteins (PTEX) has been established as the machinery responsible for the translocation of all classes of exported proteins beyond the parasitophorous vacuolar membrane of the intraerythrocytic malaria parasite. Protein export, particularly in the asexual blood stage, is crucial for parasite survival as exported proteins are involved in remodelling the host cell, an essential process for nutrient uptake, waste removal and immune evasion. Here, we have truncated the conserved C-terminus of one of the essential PTEX components, PTEX150, in Plasmodium falciparum in an attempt to create mutants of reduced functionality. Parasites tolerated C-terminal truncations of up to 125 amino acids with no reduction in growth, protein export or the establishment of new permeability pathways. Quantitative proteomic approaches however revealed a decrease in other PTEX subunits associating with PTEX150 in truncation mutants, suggesting a role for the C-terminus of PTEX150 in regulating PTEX stability. Our analyses also reveal three previously unreported PTEX-associated proteins, namely PV1, Pf113 and Hsp70-x (respective PlasmoDB numbers; PF3D7_1129100, PF3D7_1420700 and PF3D7_0831700) and demonstrate that core PTEX proteins exist in various distinct multimeric forms outside the major complex.

Identification of potent phosphodiesterase inhibitors that demonstrate cyclic nucleotide-dependent functions in apicomplexan parasites

Apicomplexan parasites, including Plasmodium falciparum and Toxoplasma gondii, the causative agents of severe malaria and toxoplasmosis, respectively, undergo several critical developmental transitions during their lifecycle. Most important for human pathogenesis is the asexual cycle, in which parasites undergo rounds of host cell invasion, replication, and egress (exit), destroying host cell tissue in the process. Previous work has identified important roles for Protein Kinase G (PKG) and Protein Kinase A (PKA) in parasite egress and invasion, yet little is understood about the regulation of cyclic nucleotides, cGMP and cAMP, that activate these enzymes. To address this, we have focused upon the development of inhibitors of 3',5'-cyclic nucleotide phosphodiesterases (PDEs) to block the breakdown of cyclic nucleotides. This was done by repurposing human PDE inhibitors noting various similarities of the human and apicomplexan PDE binding sites. The most potent inhibitors blocked the in vitro proliferation of P. falciparum and T. gondii more potently than the benchmark compound zaprinast. 5-Benzyl-3-isopropyl-1H-pyrazolo[4,3-d]pyrimidin-7(6H)-one (BIPPO) was found to be a potent inhibitor of recombinant P. falciparum PfPDEα and activated PKG-dependent egress of T. gondii and P. falciparum, likely by promoting the exocytosis of micronemes, an activity that was reversed by a specific Protein Kinase G inhibitor. BIPPO also promotes cAMP-dependent phosphorylation of a P. falciparum ligand critical for host cell invasion, suggesting that the compound inhibits single or multiple PDE isoforms that regulate both cGMP and cAMP levels. BIPPO is therefore a useful tool for the dissection of signal transduction pathways in apicomplexan parasites.

The Plasmodium translocon of exported proteins (PTEX) component thioredoxin-2 is important for maintaining normal blood-stage growth

Plasmodium parasites remodel their vertebrate host cells by translocating hundreds of proteins across an encasing membrane into the host cell cytosol via a putative export machinery termed PTEX. Previously PTEX150, HSP101 and EXP2 have been shown to be bona fide members of PTEX. Here we validate that PTEX88 and TRX2 are also genuine members of PTEX and provide evidence that expression of PTEX components are also expressed in early gametocytes, mosquito and liver stages, consistent with observations that protein export is not restricted to asexual stages. Although amenable to genetic tagging, HSP101, PTEX150, EXP2 and PTEX88 could not be genetically deleted in Plasmodium berghei, in keeping with the obligatory role this complex is postulated to have in maintaining normal blood-stage growth. In contrast, the putative thioredoxin-like protein TRX2 could be deleted, with knockout parasites displaying reduced grow-rates, both in vivo and in vitro, and reduced capacity to cause severe disease in a cerebral malaria model. Thus, while not essential for parasite survival, TRX2 may help to optimize PTEX activity. Importantly, the generation of TRX2 knockout parasites that display altered phenotypes provides a much-needed tool to dissect PTEX function.

Biochemical and functional analysis of two Plasmodium falciparum blood-stage 6-cys proteins: P12 and P41

The genomes of Plasmodium parasites that cause malaria in humans, other primates, birds, and rodents all encode multiple 6-cys proteins. Distinct 6-cys protein family members reside on the surface at each extracellular life cycle stage and those on the surface of liver infective and sexual stages have been shown to play important roles in hepatocyte growth and fertilization respectively. However, 6-cys proteins associated with the blood-stage forms of the parasite have no known function. Here we investigate the biochemical nature and function of two blood-stage 6-cys proteins in Plasmodium falciparum, the most pathogenic species to afflict humans. We show that native P12 and P41 form a stable heterodimer on the infective merozoite surface and are secreted following invasion, but could find no evidence that this complex mediates erythrocyte-receptor binding. That P12 and P41 do not appear to have a major role as adhesins to erythrocyte receptors was supported by the observation that antisera to these proteins did not substantially inhibit erythrocyte invasion. To investigate other functional roles for these proteins their genes were successfully disrupted in P. falciparum, however P12 and P41 knockout parasites grew at normal rates in vitro and displayed no other obvious phenotypic changes. It now appears likely that these blood-stage 6-cys proteins operate as a pair and play redundant roles either in erythrocyte invasion or in host-immune interactions.

Engineering herbicide tolerance in transgenic plants

The herbicide glyphosate is a potent inhibitor of the enzyme 5-enolpyruvylshikimate- 3-phosphate (EPSP) synthase in higher plants. A complementary DNA (cDNA) clone encoding EPSP synthase was isolated from a complementary DNA library of a glyphosate-tolerant Petunia hybrida cell line (MP4-G) that overproduces the enzyme. This cell line was shown to overproduce EPSP synthase messenger RNA as a result of a 20-fold amplification of the gene. A chimeric EPSP synthase gene was constructed with the use of the cauliflower mosaic virus 35S promoter to attain high level expression of EPSP synthase and introduced into petunia cells. Transformed petunia cells as well as regenerated transgenic plants were tolerant to glyphosate.

A newly discovered protein export machine in malaria parasites

Several hundred malaria parasite proteins are exported beyond an encasing vacuole and into the cytosol of the host erythrocyte, a process that is central to the virulence and viability of the causative Plasmodium species. The trafficking machinery responsible for this export is unknown. Here we identify in Plasmodium falciparum a translocon of exported proteins (PTEX), which is located in the vacuole membrane. The PTEX complex is ATP-powered, and comprises heat shock protein 101 (HSP101; a ClpA/B-like ATPase from the AAA+ superfamily, of a type commonly associated with protein translocons), a novel protein termed PTEX150 and a known parasite protein, exported protein 2 (EXP2). EXP2 is the potential channel, as it is the membrane-associated component of the core PTEX complex. Two other proteins, a new protein PTEX88 and thioredoxin 2 (TRX2), were also identified as PTEX components. As a common portal for numerous crucial processes, this translocon offers a new avenue for therapeutic intervention.

MSP1(19) miniproteins can serve as targets for invasion inhibitory antibodies in Plasmodium falciparum provided they contain the correct domains for cell surface trafficking

Antibodies from malaria-exposed individuals can agglutinate merozoites released from Plasmodium schizonts, thereby preventing them from invading new erythrocytes. Merozoite coat proteins attached to the plasma membrane are major targets for host antibodies and are therefore considered important malaria vaccine candidates. Prominent among these is the abundant glycosylphosphatidylinositol (GPI)-anchored merozoite surface protein 1 (MSP1) and particularly its C-terminal fragment (MSP1(19)) comprised of two epidermal growth factor (EGF)-like modules. In this paper, we revisit the role of agglutination and immunity using transgenic fluorescent marker proteins. We describe expression of heterologous MSP1(19)'miniproteins' on the surface of Plasmodium falciparum merozoites. To correctly express these proteins, we determined that GPI-anchoring and the presence of a signal sequence do not allow default export of proteins from the endoplasmic reticulum to merozoite surface and that extra sequence elements are required. The EGFs are insufficient for correct trafficking unless they are fused to additional residues that normally reside upstream of this fragment. Antibodies specifically targeting the surface-expressed miniprotein can inhibit erythrocyte invasion in vitro despite the presence of endogenous MSP1. Using a line expressing a green fluorescent protein-MSP1 fusion protein, we demonstrate that one mode of inhibition by antibodies targeting the MSP1(19) domain is the rapid agglutinating of merozoites prior to erythrocyte attachment.

Identification of protein complexes in detergent-resistant membranes of Plasmodium falciparum schizonts

Merozoite surface proteins of the human malaria parasite Plasmodium falciparum are involved in initial contact with target erythrocytes, a process that begins a cascade of events required for successful invasion of these cells. In order to identify complexes that may play a role in invasion we purified detergent-resistant membranes (DRMs), known to be enriched in merozoite surface proteins, and used blue native-polyacrylamide gel electrophoresis (BN-PAGE) to isolate high molecular weight complexes for identification by mass spectrometry. Sixty-two proteins were detected and these mostly belonged to expected DRM proteins classes including GPI-anchored, multi-membrane spanning and rhoptry proteins. Proteins from seven known complexes were identified including MSP-1/7, the low (RAP1/2 and RAP1/3), and high (RhopH1/H2/H3) molecular weight rhoptry complexes, and the invasion motor complex (GAP45/GAP50/myosinA). Remarkably, a large proportion of identified spectra were derived from only 4 proteins: the GPI-anchored proteins MSP-1 and Pf92, the putative GPI-anchored protein Pf113 and RAP-1, the core component of the two RAP complexes. Each of these proteins predominated in high molecular weight species suggesting their aggregation in much larger complexes than anticipated. To demonstrate that the procedure had isolated novel complexes we focussed on MSP-1, which predominated as a distinct species at approximately 500 kDa by BN-PAGE, approximately twice its expected size. Chemical cross-linking supports the existence of a stable MSP-1 oligomer of approximately 500 kDa, probably comprising a highly stable homodimeric species. Our observations also suggests that oligomerization of MSP-1 is likely to occur outside the C-terminal epidermal growth factor (EGF)-like domains. Confirmation of MSP-1 oligomerization, together with the isolation of a number of known complexes by BN-PAGE, makes it highly likely that novel interactions occur amongst members of this proteome.

A set of glycosylphosphatidyl inositol-anchored membrane proteins of Plasmodium falciparum is refractory to genetic deletion

Targeted gene disruption has proved to be a powerful approach for studying the function of important ligands involved in erythrocyte invasion by the extracellular merozoite form of the human malaria parasite, Plasmodium falciparum. Merozoite invasion proceeds via a number of seemingly independent alternate pathways, such that entry can proceed with parasites lacking particular ligand-receptor interactions. To date, most focus in this regard has been on single-pass (type 1) membrane proteins that reside in the secretory organelles. Another class of merozoite proteins likely to include ligands for erythrocyte receptors are the glycosylphosphatidyl inositol (GPI)-anchored membrane proteins that coat the parasite surface and/or reside in the apical organelles. Several of these are prominent vaccine candidates, although their functions remain unknown. Here, we systematically attempted to disrupt the genes encoding seven of the known GPI-anchored merozoite proteins of P. falciparum by using a double-crossover gene-targeting approach. Surprisingly, and in apparent contrast to other merozoite antigen classes, most of the genes (six of seven) encoding GPI-anchored merozoite proteins are refractory to genetic deletion, with the exception being the gene encoding merozoite surface protein 5 (MSP-5). No distinguishable growth rate or invasion pathway phenotype was detected for the msp-5 knockout line, although its presence as a surface-localized protein was confirmed.

Distinct protein classes including novel merozoite surface antigens in Raft-like membranes of Plasmodium falciparum

Glycosylphosphatidylinositol (GPI)-anchored proteins coat the surface of extracellular Plasmodium falciparum merozoites, of which several are highly validated candidates for inclusion in a blood-stage malaria vaccine. Here we determined the proteome of gradient-purified detergent-resistant membranes of mature blood-stage parasites and found that these membranes are greatly enriched in GPI-anchored proteins and their putative interacting partners. Also prominent in detergent-resistant membranes are apical organelle (rhoptry), multimembrane-spanning, and proteins destined for export into the host erythrocyte cytosol. Four new GPI-anchored proteins were identified, and a number of other novel proteins that are predicted to localize to the merozoite surface and/or apical organelles were detected. Three of the putative surface proteins possessed six-cysteine (Cys6) motifs, a distinct fold found in adhesive surface proteins expressed in other life stages. All three Cys6 proteins, termed Pf12, Pf38, and Pf41, were validated as merozoite surface antigens recognized strongly by antibodies present in naturally infected individuals. In addition to the merozoite surface, Pf38 was particularly prominent in the secretory apical organelles. A different cysteine-rich putative GPI-anchored protein, Pf92, was also localized to the merozoite surface. This insight into merozoite surfaces provides new opportunities for understanding both erythrocyte invasion and anti-parasite immunity.

Plasmodium falciparum merozoite surface protein 8 is a ring-stage membrane protein that localizes to the parasitophorous vacuole of infected erythrocytes

To date, the following seven glycosylphosphatidylinositol (GPI)-anchored merozoite antigens have been described in Plasmodium falciparum: merozoite-associated surface protein 1 (MSP-1), MSP-2, MSP-4, MSP-5, MSP-8, MSP-10, and the rhoptry-associated membrane antigen. Of these, MSP-1, MSP-8, and MSP-10 possess a double epidermal growth factor (EGF)-like domain at the C terminus, and these modules are considered potential targets of protective immunity. In this study, we found that surprisingly, P. falciparum MSP-8 is transcribed and translated in the ring stage and is absent from the surface of merozoites. MSP-8 is the only GPI-anchored protein known to be expressed at this time. It is synthesized as a mature 80-kDa protein which is rapidly processed to a C-terminal 17-kDa species that contains the double EGF module. As determined by a combination of immunofluorescence and membrane purification approaches, it appears likely that MSP-8 initially localizes to the parasite plasma membrane in the ring stage. Although the C-terminal 17-kDa fragment is present in more mature stages, at these times it is found in the food vacuole. We successfully disrupted the MSP-8 gene in P. falciparum, a process that validated the specificity of the antibodies used in this study and also demonstrated that MSP-8 does not play a role essential to maintenance of the erythrocyte cycle. This finding, together with the observation that MSP-8 is exclusively intracellular, casts doubt over the viability of this antigen as a vaccine. However, it is still possible that MSP-8 is involved in an early parasitophorous vacuole function that is significant for pathogenesis in the human host.

A subset of Plasmodium falciparum SERA genes are expressed and appear to play an important role in the erythrocytic cycle

The Plasmodium falciparum serine repeat antigen (SERA) has shown considerable promise as a blood stage vaccine for the control of malaria. A related protein, SERPH, has also been described in P. falciparum. Whereas their biological role remains unknown, both proteins possess papain-like protease domains that may provide attractive targets for therapeutic intervention. Genomic sequencing has recently shown that SERA and SERPH are the fifth and sixth genes, respectively, in a cluster of eight SERA homologues present on chromosome 2. In this paper, the expression and functional relevance of these eight genes and of a ninth SERA homologue found on chromosome 9 were examined in blood stage parasites. Using reverse transcriptase-PCR and microarray approaches, we demonstrate that whereas mRNA to all nine SERA genes is synthesized late in the erythrocytic cycle, it is those genes in the central region of the chromosome 2 cluster that are substantially up-regulated at this time. Using antibodies specific to each SERA, it was apparent that SERA4 to -6, and possibly also SERA9, are synthesized in blood stage parasites. The reactivity of antibodies from malaria-immune individuals with the SERA recombinant proteins suggested that SERA2 and SERA3 are also expressed at least in some parasite populations. To examine whether SERA genes are essential to blood stage growth, each of the eight chromosome 2 SERA genes was targeted for disruption. Whereas genes at the periphery of the cluster were mostly dispensable (SERA2 and -3 and SERA7 and -8), those in the central region (SERA4 to -6) could not be disrupted. The inability to disrupt SERA4, -5, and -6 is consistent with their apparent dominant expression and implies an important role for these genes in maintenance of the erythrocytic cycle.

Tissue-specific expression of the TMV coat protein in transgenic tobacco plants affects the level of coat protein-mediated virus protection

Transgenic tobacco plants were produced that express a chimeric gene encoding the coat protein (CP) of tobacco mosaic virus (TMV) under the control of the promoter from a ribulose bisphosphate carboxylase small subunit (rbcS) gene. Plant lines expressing comparable levels of CP from the rbcS and cauliflower mosaic virus 35S promoters were compared for resistance to TMV. In whole plant assays the 35S:CP constructs gave higher resistance than the rbcS:CP constructs. On the other hand, leaf mesophyll protoplasts isolated from both plant lines were equally resistant to infection by TMV. This indicated that the difference in resistance between the lines in the whole plant assay reflects differences at the level of short- and/or long-distance spread of TMV. Therefore, we propose that the difference in tissue-specific expression between the 35S and rbcS promoters accounts for greater resistance in the plant lines that express the 35S:CP chimeric genes.

Protection against tobacco mosaic virus infection in transgenic plants requires accumulation of coat protein rather than coat protein RNA sequences

Transgenic tobacco plants which express a chimeric gene encoding the tobacco mosaic virus (TMV) coat protein (CP) and the TMV 3' untranslated region are protected against infection by TMV. In this study chimeric genes that encode the sequences representing the TMV CP subgenomic RNA, but do not produce protein (because of removal of the initiation codon), and RNA that lacks the tRNA-like sequence of the TMV 3' end were expressed in transgenic plants. Only plants that accumulated CP, regardless of the presence of absence of the 3' end of TMV-RNA, were protected against infection by TMV. The results indicate that the CP per se, rather than TMV RNA, is responsible for the resistance to infection by TMV. Furthermore, the degree of protection is dependent upon the level of accumulated CP.

Protection against tobacco mosaic virus in transgenic plants that express tobacco mosaic virus antisense RNA

Transgenic tobacco plants that express RNA sequences complementary to the tobacco mosaic virus (TMV) coat protein (CP) coding sequence with or without the tRNA-like structure at the 3' end of the TMV RNA were produced. Progeny of self-pollinated plants were challenged with TMV to determine their resistance to infection. Plants that expressed RNA sequences complementary to the CP coding region and the 3' untranslated region, including the tRNA-like sequences, were protected from infection by TMV at low levels of inoculum. However, plants that expressed RNA complementary to the CP coding sequence alone were not protected from infection. These results indicate that sequences complementary to the terminal 117 nucleotides of TMV, which include a putative replicase binding site, are responsible for the protection. However, the level of protection in these plants was considerably less than in transgenic plants that expressed the TMV CP gene and accumulated CP. Since the mechanisms of protection in the two systems are different, it may be possible to increase protection by introducing both sequences into transgenic plants.

Expression of alfalfa mosaic virus coat protein gene confers cross-protection in transgenic tobacco and tomato plants

A chimeric gene encoding the alfalfa mosaic virus (AlMV) coat protein was constructed and introduced into tobacco and tomato plants using Ti plasmid-derived plant transformation vectors. The progeny of the self-fertilized transgenic plants were significantly delayed in symptom development and in some cases completely escaped infection after inoculated with AlMV. The inoculated leaves of the transgenic plants had significantly reduced numbers of lesions and accumulated substantially lower amounts of coat protein due to virus replication than the control plants. These results show that high level expression of the chimeric viral coat protein gene confers protection against AlMV, which differs from other plant viruses in morphology, genome structure, gene expression strategy and early steps in viral replication. Based on our results with AlMV and those reported earlier for tobacco mosaic virus, it appears that genetically engineered cross-protection may be a general method for preventing viral disease in plants.

Comparison of cauliflower mosaic virus 35S and nopaline synthase promoters in transgenic plants

We have compared the level of expression of the Cauliflower Mosaic Virus 35S promoter and the nopaline synthase promoter when fused to a common reporter gene. A cassette containing the neomycin phosphotransferase (type II) coding sequence followed by the nopaline synthase 3' nontranslated region was used for transcriptional and translational evaluation of the two different promoters. These chimeric genes were introduced into petunia plants and the copy number of the gene, the steady state level of NPTII transcript and the levels of NPTII enzyme activity were determined. In this paper, we report that the NPT II transcript levels are on the average 30 fold higher in plants containing CaMV 35S promoter and leader sequences than in plants containing the same reporter gene but nopaline synthase promoter and leader sequences. Similarly, plants containing the CaMV 35S promoter had an average of 110 fold higher levels of NPTII enzyme activity than those containing the nopaline synthase promoter. The significance of these results for expression of foreign genes in plants is discussed. In addition, we describe the construction of a convenient plant expression cassette vector (pMON316) which utilizes the CaMV 35S promoter.

In vitro transformation of petunia cells by an improved method of co-cultivation with A. tumefaciens strains

A method (termed co-cultivation) for transforming plant cells in vitro with A. tumefaciens strains, which was originally developed by Marton et al. (1978) Nature 277: 129-131, has been modified by the incorporation of a novel feeder plate culture system and been extended to use with petunia protoplasts. Using efficient cell plating and selection conditions for phytohormone-independent growth, large numbers of independent transformed calli can be obtained efficiently (∼10(-1)) and in less than 3 weeks following protoplast isolation. Southern hybridization analysis has confirmed that the majority of the resulting in vitro transformants contain a single copy of full length T-DNA.The high efficiency of this procedure allows simple screening to identify plant cells transformed by Ti plasmids attenuated by deletion of internal T-DNA regions. Results are presented that demonstrate the co-cultivation method can be used in conjunction with short term assays for monitoring plant gene expression.

Inheritance of functional foreign genes in plants

Morphologically normal plants were regenerated from Nicotiana plumbaginifolia cells transformed with an Agrobacterium tumefaciens strain containing a tumor-inducing plasmid with a chimeric gene for kanamycin resistance. The presence of the chimeric gene in regenerated plants was demonstrated by Southern hybridization analysis, and its expression in plant tissues was confirmed by the ability of leaf segments to form callus on media containing kanamycin at concentrations that were normally inhibitory. Progeny derived from several transformed plants inherited the foreign gene in a Mendelian manner.

Introduction of cloned human papillomavirus genomes into mouse cells and expression at the RNA level

The entire DNA genomes of five different human papillomaviruses (HPVs) were cloned into the BamHI site of pBR322 (HPV-1a, HPV-3, HPV-4, and HPV-9) or the EcoRI site of pBR325 (HPV-2), using as starting materials virus preparations isolated from papillomas of individual patients. Under stringent hybridization conditions (Tm-28 degrees), the five cloned HPVs exhibited less than 10% homology with one another. To establish model cell systems that may be useful for the identification of HPV genes and HPV gene products, mouse thymidine kinase negative (tk-) cells were cotransformed to the tk+ phenotype with the herpesvirus thymidine kinase gene and each of the five HPV cloned DNAs (either as intact recombinants or excised HPV DNA without removal of pBR). In most tk+ cell clones, a complex pattern of multiple high molecular weight inserts of HPV DNA were present in high copy number. Most of the HPV DNA sequences in the cotransformed cells were not present as unit-length episomal viral DNA. Analyses of the integration pattern (DNA blot) and RNA expression (RNA blot) of several HPV-1a and HPV-3 transformed cell lines suggest that some copies of the viral genome are integrated in a similar manner in different cell lines leading to the expression of identical viral RNA-containing species. Two of the cell lines transformed by the intact HPV-1a/pBR322 recombinant synthesized substantial amounts of four discrete viral polyadenylated cytoplasmic RNA species of 1.9, 3.2, 3.8, and 4.5 kb. Two cell lines transformed by the intact HPV-3/pBR322 recombinant synthesized 4-5 polyadenylated cytoplasmic viral RNA species ranging from 0.8 to 4.6 kb. The analysis shows that each viral RNA species appears to be a hybrid RNA molecule containing both HPV and pBR322 sequences. Based on these findings and the molecular organization of the HPV-1a genome (O. Danos, M. Katinka, and M. Yaniv (1982). EMBO J. 1, 231-237), it is possible that transcription of each of the HPV-1a RNA species is initiated using the HPV early promoter and terminated in pBR322.

Expression of bacterial genes in plant cells

Chimeric bacterial genes conferring resistance to aminoglycoside antibiotics have been inserted into the Agrobacterium tumefaciens tumor-inducing (Ti) plasmid and introduced into plant cells by in vitro transformation techniques. The chimeric genes contain the nopaline synthase 5' and 3' regulatory regions joined to the genes for neomycin phosphotransferase type I or type II. The chimeric genes were cloned into an intermediate vector, pMON120, and inserted into pTiB6S3 by recombination and then introduced into petunia and tobacco cells by cocultivating A. tumefaciens cells with protoplast-derived cells. Southern hybridization was used to confirm the presence of the chimeric genes in the transformed plant tissues. Expression of the chimeric genes was determined by the ability of the transformed cells to proliferate on medium containing normally inhibitory levels of kanamycin (50 micrograms/ml) or other aminoglycoside antibiotics. Plant cells transformed by wild-type pTiB6S3 or derivatives carrying the bacterial neomycin phosphotransferase genes with their own promoters failed to grow under these conditions. The significance of these results for plant genetic engineering is discussed.

Isolation of a human papillomavirus from a patient with epidermodysplasia verruciformis: presence of related viral DNA genomes in human urogenital tumors

The DNA genome of a human papillomavirus (HPV), tentatively designated HPV-EV, was molecularly cloned from hand to leg lesions of a patient with epidermodysplasia verruciformis, a chronic skin disease associated with a 30% risk of developing cancer. Using stringent hybridization conditions, we observed less than 5% homology between HPV-EV and the cloned genomes of HPV-1, HPV-4, HPV-5, and HPV-5a. HPV-EV DNA showed approximately 6% homology with HPV-2 and 36% homology with HPV-3. These data suggest that HPV-EV is partially related to HPV-3. Using 32P-labeled cloned HPV-EV as probe in Southern blot hybridization experiments, we detected HPV-EV-related DNA in the carcinoma in situ (Bowenoid lesion) of the vulva of the patient from which HPV-EV was isolated. HPV-EV-related DNA was detected in 2 of 10 vulva carcinomas and in 2 of 31 cervical carcinomas. Related DNA sequences were found in papillomas from each of two patients with condyloma acuminata (anogenital warts), which is of interest considering that condylomas have been reported to convert occasionally to carcinomas. The positive vulva DNAs were also probed with other cloned HPV DNAs: HPV-1, HPV-4, and HPV-5a-related sequences were not detected; HPV-3 and HPV-2 DNA probes detected strong and weak DNA bands, respectively, of the same size as found with HPV-EV. The HPV DNA sequences were present in the positive tumors mainly as free viral DNA molecules; no evidence for integration into cellular DNA was found. The emerging biological picture with papillomaviruses is that cells transformed by these viruses are maintained in a transformed state by free episomal genomes. Thus, our findings are consistent with the idea, but by no means establish, that HPVs play a role in human cancer by a similar mechanism.

Determination of coumarin and umbelliferone mixtures in whole blood by spectrophotofluorometry

A spectrophotofluorometric method is described for the quantitative analysis of coumarin, umbelliferone, and mixtures thereof in whole blood. The two drugs were selectively isolated from blood by solvent extraction. Analysis of the isolated coumarin was based on the measurement of the fluorophore at activation and emission wavelengths of 361 and 491 nm, respectively. The fluorophore was obtained by irradiating an alkaline methanolic solution of the drug with UV light. A linear relationship between fluorescence and concentration existed over the concentration range of 0.02-0.2 mug of coumarin/ml. A mean recovery value of 94.8% was obtained from whole blood. The isolated umbelliferone was determined according to established methods at activation and emission wavelengths of 370 and 450 nm, respectively, and the limit of detection was 10 times more sensitive than previously reported. A linearity response was obtained between 1 and 10 ng of umbelliferone/ml. Good recovery data for mixtures of coumarin and umbelliferone in whole blood were obtained.