Two species of full-length cDNA are synthesized in high yield by melittin-treated avian retrovirus particles

Immunomodulatory activities and biomedical applications of melittin and its recent advances

Melittin (MLT), a peptide containing 26 amino acids, is a key constituent of bee venom. It comprises ∼40%-60% of the venom's dry weight and is the main pricing index for bee venom, being the causative factor of pain. The unique properties of MLT extracted from bee venom have made it a very valuable active ingredient in the pharmaceutical industry as this cationic and amphipathic peptide has propitious effects on human health in diverse biological processes. It has the ability to strongly impact the membranes of cells and display hemolytic activity with anticancer characteristics. However, the clinical application of MLT has been limited by its severe hemolytic activity, which poses a challenge for therapeutic use. By employing more efficient mechanisms, such as modifying the MLT sequence, genetic engineering, and nano-delivery systems, it is anticipated that the limitations posed by MLT can be overcome, thereby enabling its wider application in therapeutic contexts. This review has outlined recent advancements in MLT's nano-delivery systems and genetically engineered cells expressing MLT and provided an overview of where the MLTMLT's platforms are and where they will go in the future with the challenges ahead. The focus is on exploring how these approaches can overcome the limitations associated with MLT's hemolytic activity and improve its selectivity and efficacy in targeting cancer cells. These advancements hold promise for the creation of innovative and enhanced therapeutic approaches based on MLT for the treatment of cancer.

The HIV-1 capsid and reverse transcription

The viral capsid plays a key role in HIV-1 reverse transcription. Recent studies have demonstrated that the small molecule IP6 dramatically enhances reverse transcription in vitro by stabilizing the viral capsid. Reverse transcription results in marked changes in the biophysical properties of the capsid, ultimately resulting in its breakage and disassembly. Here we review the research leading to these advances and describe hypotheses for capsid-dependent HIV-1 reverse transcription and a model for reverse transcription-primed HIV-1 uncoating.

Applications and evolution of melittin, the quintessential membrane active peptide

Melittin, the main venom component of the European Honeybee, is a cationic linear peptide-amide of 26 amino acid residues with the sequence: GIGAVLKVLTTGLPALISWIKRKRQQ-NH₂. Melittin binds to lipid bilayer membranes, folds into amphipathic α-helical secondary structure and disrupts the permeability barrier. Since melittin was first described, a remarkable array of activities and potential applications in biology and medicine have been described. Melittin is also a favorite model system for biophysicists to study the structure, folding and function of peptides and proteins in membranes. Melittin has also been used as a template for the evolution of new activities in membranes. Here we overview the rich history of scientific research into the many activities of melittin and outline exciting future applications.

The Host Cell Metabolite Inositol Hexakisphosphate Promotes Efficient Endogenous HIV-1 Reverse Transcription by Stabilizing the Viral Capsid

HIV-1 infection requires reverse transcription of the viral genome. While much is known about the biochemistry of reverse transcription from simplified biochemical reactions, reverse transcription during infection takes place within a viral core. However, endogenous reverse transcription reactions using permeabilized HIV-1 virions or purified viral cores have been inefficient. Using viral cores purified from infectious HIV-1 particles, we show that efficient reverse transcription is achieved in vitro by addition of the capsid-stabilizing metabolite inositol hexakisphosphate. The enhancement of reverse transcription was linked to the capsid-stabilizing effect of the compound, consistent with the known requirement for an intact or semi-intact viral capsid for HIV-1 infection. Our results establish a biologically relevant system for dissecting the function of the viral capsid and its disassembly during reverse transcription. The system should also prove useful for mechanistic studies of capsid-targeting antiviral drugs.

A defining activity of retroviruses is reverse transcription, the process by which the viral genomic RNA is converted into the double-stranded DNA required for virus replication. Reverse transcriptase (RT), the viral enzyme responsible for this process, was identified in 1970 by assaying permeabilized retrovirus particles for DNA synthesis in vitro. Such reactions are inefficient, with only a small fraction of viral genomes being converted to full-length double-stranded DNA molecules, possibly owing to disruption of the structure of the viral core. Here, we show that reverse transcription in purified HIV-1 cores is enhanced by the addition of the capsid-binding host cell metabolite inositol hexakisphosphate (IP6). IP6 potently enhanced full-length minus-strand synthesis, as did hexacarboxybenzene (HCB), which also stabilizes the HIV-1 capsid. Both IP6 and HCB stabilized the association of the viral CA and RT proteins with HIV-1 cores. In contrast to the wild type, cores isolated from mutant HIV-1 particles containing intrinsically hyperstable capsids exhibited relatively efficient reverse transcription in the absence of IP6, further indicating that the compound promotes reverse transcription by stabilizing the viral capsid. We also observed that the capsid-destabilizing antiviral compound PF74 inhibited endogenous reverse transcription with a potency that mirrors its ability to inhibit reverse transcription during infection. Our results show that the stabilization of the HIV-1 capsid permits efficient reverse transcription in HIV-1 cores, providing a sensitive experimental system for analyzing the functions of viral and host cell molecules and the role of capsid disassembly (uncoating) in the process.

Reconstitution and visualization of HIV-1 capsid-dependent replication and integration in vitro

During the first half of the viral life cycle, HIV-1 reverse transcribes its RNA genome and integrates the double-stranded DNA copy into a host cell chromosome. Despite progress in characterizing and inhibiting these processes, in situ mechanistic and structural studies remain challenging. This is because these operations are executed by individual viral preintegration complexes deep within cells. We therefore reconstituted and imaged the early stages of HIV-1 replication in a cell-free system. HIV-1 cores released from permeabilized virions supported efficient, capsid-dependent endogenous reverse transcription to produce double-stranded DNA genomes, which sometimes looped out from ruptured capsid walls. Concerted integration of both viral DNA ends into a target plasmid then proceeded in a cell extract-dependent reaction. This reconstituted system uncovers the role of the capsid in templating replication.

Melittin: a venom-derived peptide with promising anti-viral properties

Despite tremendous advances in the development of anti-viral therapeutics, viral infections remain a chief culprit accounting for ongoing morbidity and mortality worldwide. Natural products, in particular animal venoms, embody a veritable cornucopia of exotic constituents, suggesting an immensurable source of anti-infective drugs. In this context, melittin, the principal constituent in the venom of the European honeybee Apis mellifera, has been demonstrated to exert anti-cancer, anti-inflammatory, anti-diabetic, anti-infective, and adjuvant properties. To our knowledge, there is no review appertaining to effects of melittin against viruses, prompting us to synopsize experimental investigations on its anti-viral activity throughout the past decades. Accumulating evidence indicates that melittin curbs infectivity of a diverse array of viruses including coxsackievirus, enterovirus, influenza A viruses, human immunodeficiency virus (HIV), herpes simplex virus (HSV), Junín virus (JV), respiratory syncytial virus (RSV), vesicular stomatitis virus (VSV), and tobacco mosaic virus (TMV). However, medication safety, different routes of administrations, and molecular mechanisms behind the anti-viral activity of melittin should be scrutinized in future studies.

Finding, Conducting, and Nurturing Science: A Virologist's Memoir

My laboratory investigations have been driven by an abiding interest in understanding the consequences of genetic rearrangement in evolution and disease, and in using viruses to elucidate fundamental mechanisms in biology. Starting with bacteriophages and moving to the retroviruses, my use of the tools of genetics, molecular biology, biochemistry, and biophysics has spanned more than half a century-from the time when DNA structure was just discovered to the present day of big data and epigenetics. Both riding and contributing to the successive waves of technology, my laboratory has elucidated fundamental mechanisms in DNA replication, repair, and recombination. We have made substantial contributions in the area of retroviral oncogenesis, delineated mechanisms that control retroviral gene expression, and elucidated critical details of the structure and function of the retroviral enzymes-reverse transcriptase, protease, and integrase-and have had the satisfaction of knowing that the fundamental knowledge gained from these studies contributed important groundwork for the eventual development of antiviral drugs to treat AIDS. While pursuing laboratory research as a principal investigator, I have also been a science administrator-moving from laboratory head to department chair and, finally, to institute director. In addition, I have undertaken a number of community service, science-related "extracurricular" activities during this time. Filling all of these roles, while being a wife and mother, has required family love and support, creative management, and, above all, personal flexibility-with not too much long-term planning. I hope that this description of my journey, with various roles, obstacles, and successes, will be both interesting and informative, especially to young female scientists.

Strand transfer and elongation of HIV-1 reverse transcription is facilitated by cell factors in vitro

Recent work suggests a role for multiple host factors in facilitating HIV-1 reverse transcription. Previously, we identified a cellular activity which increases the efficiency of HIV-1 reverse transcription in vitro. Here, we describe aspects of the activity which shed light on its function. The cellular factor did not affect synthesis of strong-stop DNA but did improve downstream DNA synthesis. The stimulatory activity was isolated by gel filtration in a single fraction of the exclusion volume. Velocity-gradient purified HIV-1, which was free of detectable RNase activity, showed poor reverse transcription efficiency but was strongly stimulated by partially purified cell proteins. Hence, the cell factor(s) did not inactivate an RNase activity that might degrade the viral genomic RNA and block completion of reverse transcription. Instead, the cell factor(s) enhanced first strand transfer and synthesis of late reverse transcription suggesting it stabilized the reverse transcription complex. The factor did not affect lysis of HIV-1 by Triton X-100 in the endogenous reverse transcription (ERT) system, and ERT reactions with HIV-1 containing capsid mutations, which varied the biochemical stability of viral core structures and impeded reverse transcription in cells, showed no difference in the ability to be stimulated by the cell factor(s) suggesting a lack of involvement of the capsid in the in vitro assay. In addition, reverse transcription products were found to be resistant to exogenous DNase I activity when the active fraction was present in the ERT assay. These results indicate that the cell factor(s) may improve reverse transcription by facilitating DNA strand transfer and DNA synthesis. It also had a protective function for the reverse transcription products, but it is unclear if this is related to improved DNA synthesis.

APOBEC3G inhibits elongation of HIV-1 reverse transcripts

APOBEC3G (A3G) is a host cytidine deaminase that, in the absence of Vif, restricts HIV-1 replication and reduces the amount of viral DNA that accumulates in cells. Initial studies determined that A3G induces extensive mutation of nascent HIV-1 cDNA during reverse transcription. It has been proposed that this triggers the degradation of the viral DNA, but there is now mounting evidence that this mechanism may not be correct. Here, we use a natural endogenous reverse transcriptase assay to show that, in cell-free virus particles, A3G is able to inhibit HIV-1 cDNA accumulation not only in the absence of hypermutation but also without the apparent need for any target cell factors. We find that although reverse transcription initiates in the presence of A3G, elongation of the cDNA product is impeded. These data support the model that A3G reduces HIV-1 cDNA levels by inhibiting synthesis rather than by inducing degradation.

Role of Vif in stability of the human immunodeficiency virus type 1 core

The Vif protein of human immunodeficiency virus type 1 (HIV-1) is important for virion infectivity. Previous studies have shown that vif-defective virions exhibit structural abnormalities in the virus core and are defective in the ability to complete proviral DNA synthesis in acutely infected cells. We developed novel assays to assess the relative stability of the core in HIV-1 virions. Using these assays, we examined the role of Vif in the stability of the HIV-1 core. The integrity of the core was examined following virion permeabilization or removal of the lipid envelope and treatment with various triggers, including S100 cytosol, deoxynucleoside triphosphates, detergents, NaCl, and buffers of different pH to mimic aspects of the uncoating and disassembly process which occurs after virus entry but preceding or during reverse transcription. vif mutant cores were more sensitive to disruption by all triggers tested than wild-type cores, as determined by endogenous reverse transcriptase (RT) assays, biochemical analyses, and electron microscopy. RT and the p7 nucleocapsid protein were released more readily from vif mutant virions than from wild-type virions, suggesting that the internal nucleocapsid is less stably packaged in the absence of Vif. Purified cores could be isolated from wild-type but not vif mutant virions by sedimentation through detergent-treated gradients. These results demonstrate that Vif increases the stability of virion cores. This may permit efficient viral DNA synthesis by preventing premature degradation or disassembly of viral nucleoprotein complexes during early events after virus entry.

RNA dimerization defect in a Rous sarcoma virus matrix mutant

The retrovirus matrix (MA) sequence of the Gag polyprotein has been shown to contain functions required for membrane targeting and binding during particle assembly and budding. Additional functions for MA have been proposed based on the existence of MA mutants in Rous sarcoma virus (RSV), murine leukemia virus, human immunodeficiency virus type 1, and human T-cell leukemia virus type 1 that lack infectivity even though they release particles of normal composition. Here we describe an RSV MA mutant with a surprising and previously unreported phenotype. In the mutant known as Myr1E, the small membrane-binding domain of the Src oncoprotein has been added as an N-terminal extension of Gag. While Myr1E is not infectious, full infectivity can be reestablished by a single amino acid substitution in the Src sequence (G2E), which eliminates the addition of myristic acid and the membrane-binding capacity of this foreign sequence. The presence of myristic acid at the N terminus of the Myr1E Gag protein does not explain its replication defect, because other myristylated derivatives of RSV Gag are fully infectious (e.g., Myr2 [C. R. Erdie and J. W. Wills, J. Virol. 64:5204-5208, 1990]). Biochemical analyses of Myr1E particles reveal that they contain wild-type levels of the Gag cleavage products, Env glycoproteins, and reverse transcriptase activity when measured on an exogenous template. Genomic RNA incorporation appears to be mildly reduced compared to the wild-type level. Unexpectedly, RNA isolated from Myr1E particles is monomeric when analyzed on nondenaturing Northern blots. Importantly, the insertional mutation does not lie within previously identified dimer linkage sites. In spite of the dimerization defect, the genomic RNA from Myr1E particles serves efficiently as a template for reverse transcription as measured by an endogenous reverse transcriptase assay. In marked contrast, after infection of avian cells, the products of reverse transcription are nearly undetectable. These findings might be explained either by the loss of a normal function of MA needed in the formation or stabilization of RNA dimers or by the interference in such events by the mutant MA molecules. It is possible that Myr1E viruses package a single copy of viral RNA.

Quantitative analysis of the endogenous reverse transcriptase reactions of HIV type 1 variants with decreased susceptibility to azidothymidine and nevirapine

A large number of nucleoside analog and nonnucleoside inhibitors of HIV-1 reverse transcriptase (RT) have been developed for clinical use. Data confirm that resistant variants of HIV-1 rapidly emerge in response to the selective pressure of treatment with these agents. Detection of drug resistance generally involves detection of specific mutations in the viral genome or demonstrating a failure of the drug to suppress virus replication in culture. We have developed a PCR-based method to quantitatively examine HIV-1 DNA synthesis in vitro in endogenous reverse transcription reactions and tested it as a method to detect resistance to RT inhibitors. Under certain conditions, we were able to distinguish HIV strains with high-level resistance to azidothymidine triphosphate inhibition from sensitive strains. This method was quite useful as an assay to detect resistance to nevirapine, a nonnucleoside RT inhibitor; in reconstruction experiments, nevirapine-resistant virus was detectable when it represented 10 to 25% of the total amount of virus present in reaction mixtures. These data are examined in the light of current models of the mechanisms of action of nucleoside nonnucleoside RT inhibitors. This assay may be useful for detecting the emergence of drug-resistant HIV-1 variants during therapy.

Minus-strand DNA is present within murine type C ecotropic retroviruses prior to infection

Viral minus-strand DNA has been identified within ecotropic murine retroviruses prior to infection. The abundance of minus-strand DNA is inversely proportional to the distance from the primer binding site, suggesting that viral DNA is synthesized by reverse transcriptase with the genomic RNA as template. These findings demonstrate that replication of the retroviral genome is not initiated by infection and may begin after activation of reverse transcriptase by gag-pol cleavage during virus assembly.

Equine infectious anemia virus and human immunodeficiency virus DNA synthesis in vitro: characterization of the endogenous reverse transcriptase reaction

The endogenous reverse transcriptase reaction of equine infectious anemia virus (EIAV) has been studied, and conditions allowing synthesis of full-length minus-strand DNA have been determined. In contrast to results reported for other retroviruses, synthesis of EIAV full-length minus-strand DNA was not impaired by high concentrations of Nonidet P-40, a nonionic detergent used to make the virion envelope permeable. All components of the reaction were titrated for maximum synthesis of complete minus strands, and a time course under the standardized conditions was determined. Minor subgenomic bands were observed in some cases, and both the size and proportion varied with reaction conditions. Conditions established for full-length EIAV DNA synthesis also allowed full-genome-length human immunodeficiency virus type 1 DNA synthesis. The human immunodeficiency virus type 1 DNA product contained a greater proportion of reverse transcripts that were shorter than the complete virus genome. Also in contrast to EIAV, the endogenous synthesis of high-molecular-weight human immunodeficiency virus type 1 DNA was drastically reduced at Nonidet P-40 concentrations above 0.02%. These results indicated that a detergent-stable core is not a property shared by all lentiviruses. The EIAV virion synthetic machinery is unusually stable and provides a convenient system for further in vitro study of reverse transcription.

Avian sarcoma and leukosis virus pol-endonuclease recognition of the tandem long terminal repeat junction: minimum site required for cleavage is also required for viral growth

Integration of retroviral DNA is a site-specific reaction involving an endonuclease encoded by the viral pol gene (pol-endo). In vitro the pol-endo from avian sarcoma and leukosis viruses (ASLVs) cleaves both DNA strands near the U5-U3 junction of tandem long terminal repeats (LTR-LTR junction) in single-stranded and replicative form (RF)-I substrates. We have reported previously that the sequences that are required for cleavage of single-stranded substrates by the alpha beta form of the pol-endo differ for the plus and minus strands (G. Duyk, M. Longiaru, D. Cobrinik, R. Kowal, P. deHaseth, A. M. Skalka, and J. Leis, J. Virol. 56:589-599, 1985). This is not the case with RF-I substrates, in which a maximum of 22 base pairs of U5 and 8 base pairs of U3 were required for alpha beta pol-endo cleavage in each strand. Insertion of a palindromic octanucleotide (CATCGATG) at the LTR-LTR junction abolished cleavage in RF-I but not in single-stranded DNA substrates. Deletion of the four nucleotides (TTAA) at the junction prevented cleavage in the plus strand of RF-I DNA, but did not affect cleavage of single-stranded DNA. Furthermore, the alpha beta form of ASLV pol-endo did not recognize heterologous LTR-LTR junction sequences from the reticuloendotheliosis virus or Moloney murine leukemia virus in either substrate form, despite their sequence and structural similarities to the ASLV junction. These results support a role for a sequence-specific interaction between the ASLV pol-endo and the LTR-LTR junction domains that are required for cleavage. By using the infectious Rous sarcoma virus clone pATV8-K, we introduced a set of deletions into the U5 region that would be incorporated into the LTR-LTR junction on viral replication. In the unintegrated provirus, the deletions started 43 base pairs from the LTR-LTR junction and extended various lengths toward the junction. Results of transfection studies with these clones indicated that the U5 sequences that are required for virus production in vivo correspond to those that are required for cleavage of RF-I DNA in vitro.

Sequences near the 5' long terminal repeat of avian leukosis viruses determine the ability to induce osteopetrosis

Avian leukosis virus (ALV)-induced osteopetrosis is associated with the accumulation of unintegrated viral DNA in osteoblasts. Viruses constructed from the DNAs of an osteopetrosis-inducing ALV (Br21) and a non-osteopetrosis-inducing ALV (RAV-0) have been used to test for the role of viral genes in the induction of osteopetrosis. Our results map osteopetrotic potential to a 1,400-base-pair region near the 5' long terminal repeat. This region contains signals for the splicing, translation, and packaging of viral RNAs and coding sequences for the gag proteins p19 and p10 and the N terminus of p27.

Ca2+-dependent high-affinity complex formation between calmodulin and melittin

The amphiphatic polypeptide melittin migrates as an equimolar complex with bovine brain calmodulin when monitored by gel disc electrophoresis or gel filtration in the presence of Ca2+, even in 4M-urea. The complex disassociates in the presence of EDTA and urea. The affinity is of the same order as that of calmodulin for its target enzymes, and more than 1000-fold higher than that of calmodulin for basic peptide hormones or hydrophobic drugs. The activation of brain phosphodiesterase by calmodulin is inhibited by melittin. The kinetics of inhibition suggest competition between the enzyme and melittin for calmodulin. The calmodulin-melittin interaction may constitute a model for that existing between calmodulin and its target enzymes.

Products of reverse transcription in avian retrovirus analyzed by electron microscopy

DNA products synthesized in avian retroviral particles permeabilized with melittin have been analyzed in an electron microscope. These studies have provided further insight and subsequent refinement in the melittin activation techniques. Our electron microscope analyses verify the existence of the plus-strand single-stranded DNA branches, presumed to originate by strand-displacement synthesis (L. R. Boone and A. M. Skalka, J. Virol. 37:117-126, 1981). The branches occur at many locations along the DNA molecules and are observed at very early times, even before the minus-strand copies of the RNA genome are completed. Circular forms of different derivations are observed at early and at late times, which are possible intermediates in viral replication. Novel forms termed H structures are also described. In addition to the identification of possible intermediates, these analyses have provided further information on the sequence of events in retroviral reverse transcription. These new data are combined with previous results to generate a model of reverse transcription which incorporates strand-displacement synthesis as an essential feature.

Synthesis of murine leukemia viral DNA in vitro: evidence for plus-strand DNA synthesis at both ends of the genome

We studied the synthesis of B-tropic murine leukemia viral DNA in vitro by detergent-disrupted virions. The reaction products (detected by the Southern transfer technique) included full-length, infectious, double-stranded DNA and several subgenomic fragments. Restriction endonuclease analysis and hybridization and specific probes revealed two classes of subgenomic fragments: some were derived from the right end of the genome, and some were derived from the left end. Most of the fragments harbored one long terminal repeat copy at their ends, suggesting that they were initiated correctly. S1 nuclease and restriction endonuclease treatments of these fragments indicated that a single-stranded gap was present near the first initiation site of plus strong-stop DNA. The treatments also suggested the presence of a second initiation site flanked by a single-stranded gap 0.9 kilobase pairs from the right end of the genome. Our data clearly show that plus-strand DNA is synthesized at both ends of the genome, by using plus strong stop as the first initiation site and additional initiation sites.

Comparative restriction endonuclease maps of proviral DNA of the primate type C simian sarcoma-associated virus and gibbon ape leukemia virus group

Extrachromosomal DNA was purified from canine thymus cells acutely infected with different strains of infectious primate type C viruses of the woolly monkey (simian) sarcoma helper virus and gibbon ape leukemia virus group. All DNA preparations contained linear proviral molecules of 9.1 to 9.2 kilobases, at least some of which represent complete infectious proviral DNA. Cells infected with a replication-defective fibroblast-transforming sarcoma virus and its helper, a replication-competent nontransforming helper virus, also contained a 6.6- to 6.7-kilobase DNA. These proviral DNA molecules were digested with different restriction endonucleases, and the resultant fragments were oriented to the viral RNA by a combination of partial digestions, codigestion with more than one endonuclease, digestion of integrated proviral DNA, and hybridization with 3'- and 5'-specific viral probes. The 3'- and 5'-specific probes each hybridized to fragments from both ends of proviral DNA, indicating that, in common with those of other retroviruses, these proviruses contain a large terminal redundancy at both ends, each of which consists of sequences derived from both the 3' and 5' regions of the viral RNA. The proviral sequences are organized 3',5'-unique-3',5'. Four restriction enzymes (KpnI, SmaI, PstI, and SstI) recognized sites within the large terminal redundancies, and these sites were conserved within all the isolates tested. This suggests that both the 3' and 5' ends of the genomic RNA of these viruses are extremely closely related. In contrast, the restriction sites within the unique portion of the provirus were not strongly conserved within this group of viruses, even though they were related along most of their genomes. Whereas the 5' 60 to 70% of the RNA of these viruses was more closely related by liquid hybridization experiments than was the 3' 30 to 40%, restriction sites within this region were not preferentially conserved, suggesting that small sequence differences or point mutations or both exist throughout the entire unique portion of the genome among these viruses.

Transcription of vesicular stomatitis virus activated by pardaxin, a fish toxin that permeabilizes the virion membrane

The toxic protein, Pardaxin, of the Red Sea flatfish Pardachirus marmoratus readily induced transcription of vesicular stomatitis virus by making the virion membrane permeable to nucleoside triphosphates in the absence of nonionic detergents. Virion transcription was activated over a wide range of Pardaxin concentrations, but at optimal concentrations, the rate of transcription exceeded that induced by Triton X-100. The inhibitory effect of M protein was manifested for both Pardaxin-induced and Triton-induced transcription at high concentrations of vesicular stomatitis virions; however, unlike the Triton-induced reaction, the inhibitory effect of M protein was not reversed by polyglutamic acid added to the Pardaxin-induced transcription reaction. We propose that activation of virion transcription by Pardaxin resembles more closely intracellular transcription initiated by virion penetration than does detergent-activated transcription of vesicular stomatitis virus.

Association of the transformation-specific protein pp60src with the membrane of an avian sarcoma virus

The transformation-specific protein pp60(src) coded for by avian sarcoma viruses and its associated protein kinase activity is present in virus particles of Rous sarcoma virus, Schmidt-Ruppin strain, subgroup D. Quantitative comparison of the immunoglobulin G-phosphorylating activity in Schmidt-Ruppin D virus and Schmidt-Ruppin D virus-transformed fibroblasts indicated that there was two- to fourfold less activity in the virus particles. Disruption of virus particles with nonionic detergent demonstrated that the protein kinase activity fractionated together with the viral membrane protein gp85. Therefore, viral membranes were isolated by floating detergent-disrupted virus through a discontinuous sucrose density gradient. At a characteristic density corresponding to 26% sucrose, viral membranes were identified by the radioactively labeled viral glycoprotein and furthermore by the membrane marker enzyme Na(+)-K(+)-stimulated, Mg(2+)-activated ATPase and were visualized by electron microscopy. Contamination by cell membranes could be ruled out, since (i) the virus preparation was free of cell membrane contaminants as judged from electron microscopy, (ii) floating of intact virus did not release membraneous material, and (iii) virus-free tissue culture fluid from Schmidt-Ruppin D virus-transformed nonproducer cells (which potentially contain cell membranes) did not contribute any immunoglobulin G-phosphorylating activity after mixing with nontransforming virus and pelleting it. Both pp60(src) and the protein kinase activity were found to be associated with the viral membrane. Solubilization of virus by detergent released two phosphoproteins, with molecular weights of 42,000 and 45,000 which reacted with sera specific for pp60(src) and revealed protein kinase activity but which were not membrane bound and may have represented degradation products of pp60(src). Surface iodination of intact virus particles (harvested at 3-h intervals) did not result in radioactive labeling of pp60(src), whereas collection at 24-h intervals allowed iodination of pp60(src). In contrast to the viral glycoprotein gp85, the iodinated virion-associated pp60(src) was insensitive to mild proteolytic treatment. Binding to tumorbearing-rabbit serum, immunoglobulin G phosphorylation, and endogenous phosphorylation of 60,000-, 45,000-and 42,000-dalton proteins required lysed virus and were not possible with intact virus. These results indicated that pp60(src) was embedded within the viral membrane. Membrane proteins phosphorylated in vitro were analyzed for their phosphoamino acid composition. Eight polypeptides exhibited phosphorylation in tyrosine and were absent in nontransforming viral controls.

Viral DNA synthesized in vitro by avian retrovirus particles permeabilized with melittin. II. Evidence for a strand displacement mechanism in plus-strand synthesis

Analyses of the native DNA product of mellitin-activated avian retrovirus reverse transcription have revealed a unique structure. The vast majority of the molecules were linear, either 7.7 (genome) or 8.0 (extended genome) kilobases in length, and contained single-stranded DNA branches distributed throughout. These conclusions are based on electrophoretic properties of intact and restriction endonuclease-treated molecules before and after treatment with single-strand-specific nuclease S1. Preliminary data from linear viral DNA extracted from infected cells suggest that these molecules have a similar structure. The findings summarized in this report and those in the preceding paper indicated that the single-stranded branches are of positive polarity and are generated by a strand displacement mechanism. The existence of these branches suggests a role for strand displacement in replication and recombination.

Viral DNA synthesized in vitro by avian retrovirus particles permeabilized with melittin. I. Kinetics of synthesis and size of minus- and plus-strand transcripts

We have examined the kinetics of synthesis of minus [(-)]- and plus [(+)]-strand viral DNA in melittin-permeabilized avian retrovirus particles. The reaction was biphasic. There was a very rapid initial rate, followed, after approximately 1 h, by a lower rate. Many discrete bands of subgenomic-length (-) strands were produced after 10 and 20 min of synthesis; genome-length (7.7-kilobase [kb]) (-) strands were detected within 30 min. Extension to an 8.0-kb (-)-strand species was evident by 60 min. This extension was inhibited by actinomycin D. Synthesis of (+) strands (which is also inhibited by actinomycin D) began early, before any (-) strands were completed, and continued for more than 4 h beyond the time when synthesis of full-length DNA had terminated. Two distinct species of (+)-strand DNA, 0.27 and 0.35 kb, could be observed at the earliest times. Their presence was quickly obscured by subsequent formation of (+)-strand molecules of molecular length between 0.2 and 2.0 kb.